Lens system

ABSTRACT

A lens system composed of a first lens unit, a second lens unit having a positive refractive power, a third lens unit having a negative refractive power, and a fourth lens unit having a positive refractive power. The lens system is composed of a small number of lens components and configured compact by disposing at least one radial type gradient index lens component in the second lens unit and selecting an adequate refractive index distribution for the radial type gradient index lens component.

This is a division of application Ser. No. 08/825,134, filed Mar. 19,1997 now U.S. Pat. No. 5,973,850.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The present invention relates to a lens system which is to be used invideo cameras, TV telephones, etc. and uses a radial type gradient indexlens component.

b) Description of the Prior Art

In the recent years where there are prevailing video cameras and TVtelephones using solid-state image pickup devices such as CCDs, there isa demand to reduce the number of lens components disposed in a lenssystem which is to be used in these optical apparatuses for configuringthe optical apparatuses compactor and lowering manufacturing coststhereof.

On the other hand, it is demanded to enhance optical performance of sucha lens system as picture elements are arranged at higher densities onimage pickup devices.

A lens system disclosed by Japanese Patent Kokai Publication No. Hei7-181376, for example, is generally known as a lens system which hashigh optical performance and is usable in video cameras, TV telephones,etc. This conventional example is composed of ten to twelve lenselements so as to be a lens system having high optical performance.

Further, a lens system disclosed by Japanese Patent Kokai PublicationNo. Hei 5-134172 or Hei 6-230273 is known as another conventionalexample which uses a radial type gradient index lens component forreducing a number of lens components while maintaining high opticalperformance, and is composed of three lens components.

However, the conventional example mentioned above is configured for usein silver salt cameras and makes an amount of marginal rays insufficientwhen it is used in a lens system which uses a solid-state image pickupdevice since it has an exit pupil located extremely close to an imagesurface and does not allow nearly telecentric incidence of offaxial raysonto the image surface.

Further, a lens system disclosed by Japanese Patent Kokai PublicationNo. Hei 7-159697 is known as a conventional example of lens system whichuses a gradient index lens component and is composed of three lenscomponents. However, this conventional example is an objective lenssystem for microscopes using a radial type gradient index lens componentand has a narrow field angle.

Furthermore, a lens system disclosed by Japanese Patent KokaiPublication No. Hei 4-97309 is known as a conventional example of lenssystem which has a relatively large field angle and is configured whiletaking telecentric incidence of offaxial rays into consideration.However, this conventional example has an extremely large difference inrefractive index between an axial portion and a marginal portion of thegradient index lens component, whereby it is difficult to manufacture amaterial for the gradient index lens component.

Moreover, lens systems disclosed by Japanese Patent Kokai PublicationNo. Sho 52-29238 and No. Hei 5-107471 are known as conventional exampleseach of which is composed of two to five lens components. However, theseconventional examples are objective lens systems for endoscopes whichuse radial type gradient index lens components and produce distortion inextremely large amounts.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a compact lenssystem which is composed of lens components in a number on the order oftwo to five and has favorably corrected aberrations.

The lens system according to the present invention, which has a firstcomposition, is composed of a first lens unit which comprises at leastone positive lens element and at least one negative lens element, and asecond lens unit which comprises at least one positive lens element, andone radial type gradient index lens component having a refractive indexdistribution which is axially symmetrical in a radial direction.

The lens system according to the present invention, which has a secondcomposition, is composed of a plurality of lens components, andcomprises an aperture stop disposed on the object side and a radial typegradient index lens component which has a refractive index distributionwhich is axially symmetrical in the radial direction.

The lens system according to the present invention, which has a thirdcomposition, is composed of a first lens unit consisting of a singlelens component, an aperture stop and a second lens unit comprising atleast one positive lens component and a radial type gradient index lenscomponent having a refractive index distribution which is axiallysymmetrical in the radial direction.

Moreover, the lens system according to the present invention which has afourth composition, is composed of a plurality of lens components, usesat least one reflecting surface disposed in the lens system andcomprises at least one radial type gradient index lens component havinga refractive index distribution which is axially symmetrical in theradial direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 5 show sectional views illustrating first through fifthembodiments respectively of the lens system according to the presentinvention;

FIG. 6A shows a sectional view illustrating a sixth embodiment of thelens system according to the present invention;

FIG. 6B shows an enlarged scale view of an aperture stop in the sixthembodiment of the present invention;

FIG. 7A shows a sectional view illustrating a seventh embodiment of thelens system according to the present invention;

FIG. 7B shows an enlarged scale view of an aperture stop in the seventhembodiment of the present invention;

FIGS. 8 through 49 show sectional views illustrating eighth throughforty-ninth embodiments respectively of the lens system according to thepresent invention;

FIG. 50 shows a sectional view illustrating another example of theforty-ninth t of the present invention;

FIGS. 51A, 51B, 51C and 51D show diagrams schematically illustrating acondition where the lens system according to the present invention isassembled in an electronic image pickup camera;

FIGS. 52A and 52B show diagrams illustrating means for exchanginginterchangeable lenses in a lens system according to the presentinvention which is composed of a master lens and interchangeable lenses;

FIGS. 53A, 53B and 53C show diagrams illustrating another means forexchanging interchangeable lenses in a lens system according to thepresent invention which is composed of a master lens and interchangeablelenses; and

FIG. 54 shows a sectional view illustrating a composition of a fiftiethembodiment of the lens system according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The lens system according to the present invention is characterized inthat it is composed of a first lens unit which consists of at least onepositive lens component and at least one negative lens component, and asecond lens unit which consists of at least one positive lens component,and that the second lens unit comprises at least one radial typegradient index lens component having a refractive index distributionwhich is axially symmetrical in a radial direction expressed by aformula (a) shown below, and satisfying the following conditions (1) and(4):

    Nλ(r)=N.sub.0 λ+N.sub.1 λr.sup.2 +N.sub.2 λr.sup.4 +                                         (a)

    1/V1<1/VO                                                  (1)

    -0.5<N.sub.1 ×f.sup.2 <-0.01                         (4)

wherein the reference symbol r represents a distance as measured from anoptical axis in the radial direction, the reference symbol Nλ(r)designates a refractive index for a wavelength λ at a point located atthe distance r, the reference symbol N_(i) λ designates a refractiveindex distribution coefficient of 2i'th order at the wavelength A, thereference symbols ν_(o) and ν_(i) denote values expressed by formula (c)and (d) respectively shown below, and the reference symbol f representsa focal length of the lens system as a whole.

    V.sub.0 =(N.sub.0d -1)/(N.sub.0F -N0C)                     (c)

    ν.sub.i =N.sub.id /(N.sub.iF -N.sub.iC)                 (d)

wherein the reference symbols N_(od), N_(oF) and N_(oC) representrefractive indices on the optical axis for the d-line, F-line and C-linerespectively, and the reference symbols N_(id), N_(iF) and N_(iC)designate distribution coefficients of the 2i'th order for the d-line,F-line and C-line respectively.

In the specification of the present invention, the d-line is taken as astandard wavelength and the reference symbols such as N_(i) representvalues for the d-line unless wavelength are otherwise specified.

The lens system according to the present invention which has the firstcomposition is composed of the first lens unit having a negativerefractive power and the second lens unit having a positive refractivepower as described above, or is configured as a retrofocus type lenssystem using at least one radial type gradient index lens component. Aretrofocus type lens system is used for obtaining a lens system whichuses a solid-state image pickup device such as a CCD and has arelatively wide field angle. Though the retrofocus type lens system isadvantageous for widening the field angle and telecentric incidence, ithas an asymmetrical refractive power distribution and has a defect thatoffaxial aberration can hardly be corrected in the lens system. When thelens system is composed of two to four lens components, in particular,lateral chromatic aberration is apt to be undercorrected, thereby makingit difficult to obtain favorable imaging performance. For correcting thelateral chromatic aberration, it is desirable to use at least one radialtype gradient index lens component in the second lens unit.

Since a radial type gradient index lens component has a refractive indexdistribution which is axially symmetrical in the radial direction, it iscapable of refracting rays in a medium of the lens component and has alarger freedom for correction of aberrations. It is known that a radialtype gradient index lens component has a characteristic that it isexcellent in correction of chromatic aberration, in particular.

Lateral chromatic aberration LTC produced by a gradient index lenscomponent which has a refractive index expressed by the above-mentionedformula (a) is expressed by the following formula (b):

    LTC=K(φ.sub.s /V.sub.0 +(V.sub.1)                      (b)

wherein the reference symbol K represents a constant having a valuewhich is determined dependently on heights of offaxial rays and an angleof a final axial ray, the reference symbol φ_(s) designates a refractivepower of surface of the radial type gradient index lens componentconsidered as a thin lens and the reference symbol φ_(m) denotes arefractive power of medium of the radial type gradient index lenscomponent.

Further, it is known that the refractive power of medium φ_(m) isapproximated by the following formula (e):

    φ.sub.m ≈-2N.sub.1 t.sub.G                     (e)

wherein the reference symbol t_(G) represents thickness of the radialtype gradient index lens component.

As one can understand from the formula (b), it is possible to controlchromatic aberration produced to a desired amount by varying a value ofVI in the second term. For configuring a radial type gradient index lenscomponent so as to produce chromatic aberration in an amount smallerthan that of chromatic aberration produced by a homogeneous lenscomponent having a refractive power which is the same as that of theradial type gradient index lens component, it is required from theformula (b) to satisfy the following formula (m):

    φ.sub.s /V.sub.0 +φ.sub.m /V.sub.1 <φ.sub.h /V.sub.0(m)

The left side represents a radial type gradient index lens component,and the right side designates a homogeneous lens component, in which thereference symbol Oh denotes a refractive power of the homogeneous lenscomponent and the homogeneous lens component has an Abbe's number whichis the same as an Abbe's number of the radial type gradient index lenscomponent on the optical axis. Since the radial type gradient index lenscomponent and the homogeneous lens component have the same refractivepower, there is established the following formula (n):

    φ.sub.h =φ.sub.s +φ.sub.m                      (n)

From the two formulae (m) and (n) mentioned above, we obtain thefollowing formula (p):

    φ.sub.s /V.sub.0 +φ.sub.m /V.sub.1 <φ.sub.s /V.sub.0 +φ.sub.m /V.sub.0                                     (p)

The condition (1) mentioned above is derived from this formula (p).

The condition (1) is required for configuring a radial type gradientindex lens component so as to produce chromatic aberration in an amountsmaller than that of chromatic aberration produced by a homogeneous lenscomponent having a refractive power which is the same as that of theradial type gradient index lens component.

For correcting lateral chromatic aberration which poses a problem in thelens system according to the present invention, it is desirable toconfigure a radial type gradient index lens component so as to satisfythe condition (1) and have a positive refractive power, and dispose itin a second lens unit which has a positive refractive power. Byselecting such a composition, it is possible to configure the secondlens unit so as to produce lateral chromatic aberration in a smalleramount, thereby correcting lateral chromatic aberration favorably in thelens system as a whole.

If the condition (1) is not satisfied, lateral chromatic aberration willbe undercorrected.

For correcting lateral chromatic aberration, it is conceivable toconfigure a radial gradient index lens component so as to have anegative refractive power and dispose this lens component in a firstlens unit which has a negative refractive power. When a chromaticaberration produced by a negative lens component is reduced, however,longitudinal chromatic aberration will undesirably be undercorrected inthe lens system as a whole.

As one can understand from the formula (b), it is necessary forfavorable correction of chromatic aberration to strengthen therefractive power of medium φ_(m) to a certain degree. It is thereforenecessary to enhance N₁ or enlarge the lens thickness t_(G). However, alens component is thickened and a total length of a lens system isprolonged by extremely enlarging a value of t_(G). For this reason, itis desirable for the lens system according to the present invention thatN₁ satisfied the following condition (2):

    -2<N.sub.1 ×f.sup.2 <-0.005                          (2)

By configuring a gradient index lens component so as to satisfy thecondition (2) mentioned above, it is possible to favorably correctlateral chromatic aberration by strengthening a refractive power ofmedium without extremely thickening the lens component. If the upperlimit of -0.005 of the condition (2) is exceeded, lateral chromaticaberration will be undercorrected. If the lower limit of -2 of thecondition (2) is not satisfied, in contrast, lateral chromaticaberration will be overcorrected.

When the lens system according to the present invention is to be used ina system which requires higher imaging performance, it is desirable toconfigure the lens system so as to satisfy the following condition (3):

    -1<N.sub.1 ×f.sup.2 <-0.01                           (3)

Lateral chromatic aberration can be corrected favorably when thecondition (3) is satisfied.

If the upper limit of -0.01 of the condition (3) is exceeded, lateralchromatic aberration will be undercorrected. If the lower limit of -1 ofthe condition (3) is not satisfied, in contrast, lateral chromaticaberration will be overcorrected, or correction of lateral chromaticaberration will be insufficient for use of the lens system in a systemwhich requires high imaging performance.

When an attempt is made to configure the lens system according to thepresent invention which is composed of lens components in a small numberon the order of two to four so as to have a wide field angle, a positivePetzval's sum is enlarged, thereby producing a tendency to tilt an imagesurface toward the object side. For correcting this tendency, weconceived to correct the positive Petzval's sum using a radial typegradient index lens component.

A Petzval's sum to be produced by a radial type gradient index lenscomponent is approximated by the following formula (f):

    PTZ=φ.sub.s /N.sub.0 +φ.sub.m /N.sub.0.sup.2

Since the denominator of the second term is squared in the formula (f)mentioned above, it is possible to reduce a value of a Petzval's sum byusing a radial type gradient index lens component in place of ahomogeneous lens component having a refractive power which is the sameas that of the radial type lens component. However, a sufficient effectto correct a Petzval's sum cannot be obtained when the radial typegradient index lens component has a weak refractive power of mediumφ_(m). When the condition (2) mentioned above is satisfied, a radialtype gradient index lens component has a sufficiently strong power ofmedium, thereby being capable of correcting a Petzval's sum in the lenssystem according to the present invention. That is to say, the condition(2) mentioned above is required for favorably correcting not onlylateral chromatic aberration but also a Petzval's sum.

If the upper limit of -0.005 of the condition (2) is exceeded, aPetzval's sum will be undercorrected, thereby tilting an image surfacetoward the object side. If the lower limit of -2 of the condition (2) isnot satisfied, in contrast a Petzval's sum will be overcorrected,thereby tilting the image surface in a direction away from an object.

For correcting a Petzval's sum more favorably, it is desirable tosatisfy the above-mentioned condition (3).

If the upper limit of -0.01 of the condition (3) is exceeded, aPetzval's sum will be undercorrected, thereby tilting the image surfacetoward the object side. If the lower limit of -1 of the condition (3) isnot satisfied, a Petzval's sum will be overcorrected, therebyundesirably tilting the image surface in the direction away from theobject.

When the lens system according to the present invention is to be used ina system which has a wide field angle and requires high imagingperformance, it is desirable to configure the lens system so as tosatisfy the condition (3).

For favorably correcting lateral chromatic aberration, it is desirableto satisfy the following condition (4):

    -0.5<N.sub.1 ×f.sup.2 <-0.02                         (4)

If the upper limit of -0.02 of the condition (4) is exceeded, lateralchromatic aberration or a Petzval's sum will undesirably beundercorrected. If the lower limit of -0.5 of the condition (4) is notsatisfied, in contrast, a Petzval's sum or lateral chromatic aberrationwill be overcorrected, thereby undesirably tilting the image surface inthe direction away from the object side.

For favorably correcting lateral chromatic aberration in particular in aretrofocus type lens system with a smaller number of lens components, itis desirable to compose a first lens unit of a positive lens componentand a negative lens component. By composing a first lens unit asdescribed above, it is possible to reduce lateral chromatic aberrationto be produced in this first lens unit though it is composed only of thetwo lens components, thereby making it possible to obtain a lens systemhaving high imaging performance.

For obtaining a retrofocus type lens system which is composed of anextremely small number of lens components and can be manufactured at anextremely low cost, it is desirable to compose a first lens unit of asingle negative lens component. Further, by using a radial type gradientindex lens component in a second lens unit, it is possible to correctaberrations favorably in a lens system even when the first lens unit iscomposed of the single negative lens component.

For correcting distortion, in particular in a retrofocus type lenssystem with a smaller number of lens components, it is desirable tocompose a first lens unit of two negative lens components. By composinga first lens unit of two negative lens components, it is possible toshare a negative refractive power of the first lens unit between the twonegative lens components, thereby reducing barrel form distortionproduced in the first lens unit.

For obtaining a retrofocus type lens system which is composed of anextremely small number of lens components and can be manufactured at anextremely low cost, it is desirable to compose a second lens unit of asingle radial type gradient index lens component. By using a radial typegradient index lens component, it is possible to favorably correctaberrations even when the second lens unit is composed of a single lenscomponent, thereby obtaining a lens system composed of a small number oflens components.

For obtaining an image pickup lens system by using an image pickupdevice which has picture elements arranged at a narrow pitch with theretrofocus type lens element according to the present invention, it isdesirable to compose a second lens unit of two lens components at leastone of which is a radial type gradient index lens component. Though itis possible to compose the second lens unit of a single radial typegradient index lens component, it is desirable to use an additional lenscomponent for correcting aberrations more favorably, thereby obtaining alens system which has higher imaging performance.

The lens system according to the present invention can be configured asa lens system which is other than the retrofocus type lens system havingthe first composition, or has a composition described below (the secondcomposition). Speaking concretely, the lens system according to thepresent invention which has the second composition is copposed of aplurality of lens components including at least one radial type gradientindex lens component and comprises a stop disposed on the object side.

In a case of an optical system which uses an image pickup device such asa CCD or an optical fiber bundle on an image pickup surface, it isdesirable for preventing marginal rays from being attenuated to allowoffaxial rays to be incident onto an image surface nearlytelecentrically. When a stop is disposed on the object side as in a lenssystem as in the lens system according to the present invention whichhas the second composition, it is easy to allow offaxial rays nearlytelecentrically. Disposition of a stop on the object side describedabove provides another merit to make it relatively easy to correctlateral chromatic aberration and distortion which are produced due to alens arrangement which is asymmetric as compared with that in aretrofocus type lens system. However, the lens system according to thepresent invention which has the second composition can hardly have awide field angle as compared with the retrofocus type lens system.Therefore, it is desirable to use the lens system according to thepresent invention which has the second composition as a lens systemhaving a field angle 2(on the order of 500 to 650. Aberrations inparticular can be corrected favorably in the lens system according tothe present invention which has the second composition by using a radialtype lens component that is excellent in correction of aberrations asdescribed above.

As compared with a disposition of a stop in a lens system, thedisposition of a stop on the object side provides a merit that opticalperformance of a lens system is influenced less due to eccentricities oflens components. Though an eccentricity of a lens barrel poses a problemwhen lens components are disposed before and after a stop, such aneccentricity of a lens barrel poses no problem when a stop is disposedon the object side.

Further, it is hard to correct lateral chromatic aberration produced bypositive lens components in the lens system according to the presentinvention which has the second composition. For correcting lateralchromatic aberration favorably, it is desirable to configure the radialtype gradient index lens component so as to satisfy the condition (1)mentioned above. When the condition (1) is satisfied, the radial typegradient index lens component produces chromatic aberration in a smalleramount, thereby making it possible to correct lateral chromaticaberration favorably in the lens system as a whole. If the condition (1)is not satisfied, lateral chromatic aberration will be undercorrected inthe lens system as a whole.

Since the lens system according to the present invention which has thesecond composition comprises no lens component which has a strongnegative refractive power unlike the first lens unit of the retrofocustype lens system, a Petzval's sum is apt to be remarkable in the formerlens system as a whole.

Accordingly, it is desirable to configure the radial type gradient indexlens component to be used in the lens system which has the secondcomposition so as to satisfy the condition (2). So far as the condition(2) is satisfied, the radial type gradient index lens element has asufficient refractive power of medium and is capable of favorablycorrecting a Petzval's sum. If the upper limit of -0.005 of thecondition (2) is exceeded, a Petzval's sum will be undercorrected,thereby undesirably tilting an image surface toward the object side. Ifthe lower limit of -2 of the condition (2) is not satisfied, incontrast, a Petzval's sum will be overcorrected, thereby undesirablytilting the image surface in a direction away from an object. Also forcorrecting lateral chromatic aberration, it is desirable to satisfy thecondition (2) as described above.

The lens system according to the present invention which has the thirdcomposition is composed of a first lens unit which consists of a singlepositive lens component and a second lens unit which comprises at leastone radial type gradient index lens component.

The third composition is relatively similar to the second compositiondescribed above, or different from the second composition mainly in thata positive lens component is disposed on the object side of a stop inthe third composition.

The lens system according to the present invention is configured mainlyfor nearly telecentric incidence of offaxial rays onto an image surfaceand favorable correction of aberrations as described above. Incorrection of aberrations, it is important to correct offaxialaberrations which pose problems in a lens system which has a wide fieldangle in particular.

By disposing a positive lens component as in the lens system accordingto the present invention which has the third composition, asymmetry of alens system composed of a small number of lens components as a whole ismoderated, thereby making it possible to favorably correct offaxialaberrations such as lateral chromatic aberration and distortion.Further, the disposition of the positive lens component makes it easy toallow offaxial rays to be telecentrically incident onto the imagesurface since the stop is disposed on the image side of the first lenscomponent, or relatively on the object side. Further, it is desirable touse a radial type gradient index lens component for favorably correctingaberrations.

As compared with a retrofocus type lens system, the lens systemaccording to the present invention which has the third composition has afield angle which can be widened rather hardly. It is thereforedesirable to use this lens system with a specification for a field anglenarrower than 2ω on the order of 50° to 65°.

For correcting lateral chromatic aberration in the radial type gradientindex lens component disposed on the image side of the stop in the lenssystem according to the present invention which has the thirdcomposition, it is desirable to satisfy the condition (1). When thecondition (1) is satisfied, lateral chromatic aberration can becorrected favorably as described above. If the condition (1) is notsatisfied, lateral chromatic aberration will be undercorrected.

Further, a positive Petzval's sum is apt to be remarkable in the lenssystem according to the present invention which has the thirdcomposition since it comprises no lens component having a strongnegative refractive power, unlike the first lens unit of the retrofocustype lens system. For correcting this Petzval's sum, it is desirable toconfigure the radial type gradient index lens component so as to satisfythe condition (2). When the condition (2) is satisfied, the radial typegradient index lens element has a sufficiently strong refractive powerof medium, thereby being capable of favorably correcting the Petzval'ssum. If the upper limit of -0.005 of the condition (2) is exceeded, thePetzval's sum will be undercorrected, thereby undesirably tilting theimage surface toward the object side. If the lower limit of -2 of thecondition (2) is not satisfied, in contrast, the Petzval's sum will beovercorrected, thereby undesirably tilting the image surface in adirection away from an object. Also for correction of lateral chromaticaberration, it is desirable to satisfy the condition (2) as describedabove.

For correcting distortion in particular in the lens system according tothe present invention which has the second or third composition, it isdesirable to dispose at least two positive lens components on the imageside of the stop, thereby sharing a positive refractive power betweenthese lens components. For correcting lateral chromatic aberrationfavorably in the lens system according to the present invention whichhas the second or third composition, it is desirable to disposed atleast one positive lens component and at least one negative lenscomponent on the image side of the stop.

The lens system according to the present invention which has a fourthcomposition crises at least one reflecting surface and at least oneradial type gradient index lens component.

It is very difficult to configure a lens system so as to have a shorttotal length and high imaging performance for satisfying the demands forcompact cameras which have high optical performance. However, it ispossible to obtain an extremely compact lens system while maintaininghigh imaging performance by using a reflecting surface as a portion ofthe lens system.

FIGS. 51A, 51B, 51C and 51D exemplify an electronic image pickup camerato which the lens system according to the present invention isapplicable: FIG. 51A schematically showing the electronic image pickupcamera, FIG. 51B showing a sectional view of the lens system, FIG. 51Cshowing a case where the lens system is applied without using areflecting surface and FIG. 51D showing a case where the lens system isapplied with a reflecting surface. In these drawings, a referencenumeral 1 represents the lens system as seen from an object, a referencenumeral 2 designates an optical axis of the lens system, a referencenumeral 3 denotes an image surface, a reference numeral 4 represents alens component, a reference numeral 5 designates a frame and a referencenumeral 6 denotes a reflecting surface. Further, a reference symbol xrepresents a direction of the depth of the camera, a reference symbol ydesignates a direction of length of the camera and a reference symbol tdenotes a total length of the camera.

In recent years, cameras having sizes that are compact enough to be putinto pockets of jackets or shirts are in strong demand and it is desiredto configure thin cameras or shorten lengths in the x direction.However, total lengths of lens systems tend to be prolonged forobtaining optical systems which have high imaging performance and it isvery difficult to make compactness compatible with high opticalperformance. Accordingly, the fourth composition uses a reflectingsurface so as to satisfy the demand for thin cameras by bending anoptical axis. When a lens system which has a total length t, which isshown in FIG. 51B, is applied to a camera shown in FIG. 51A withoutusing a reflecting surface, for example, the lens system cannot beaccommodated in the x direction as shown in FIG. 51C and protrudes froma camera body. By bending the optical axis using a reflecting surface 6as shown in FIG. 51D, it is possible to accomodate the lens system inthe camera so as not to protrude in the x direction.

For correcting lateral chromatic aberration favorably in the lens systemaccording to the present invention, which has the fourth composition, itis desirable to configure the radial type gradient index lens componentso as to satisfy the condition (1). When the radial type gradient indexlens component satisfies the condition (1), it is capable of favorablycorrecting lateral chromatic aberration. If the condition (1) is notsatisfied, chromatic aberration will undesirably be undercorrected.

For correcting a Petzval's sum and lateral chromatic aberration in thelens system according to the present invention, which has the fourthcomposition, it is desirable to satisfy the condition (2). When theradial type gradient index lens component satisfies the condition (2),it has a sufficiently large value of refractive power of medium, and iscapable of favorably correcting a Petzval's sum and lateral chromaticaberration. If the upper limit of -0.005 of the condition (2) is exceed,a Petzval's sum will be undercorrected, thereby undesirably tilting animage surface toward the object side or underecorrecting lateralchromatic aberration. If the lower limit of -2 of the condition (2) isexceeded, a Petzval's sum will be overcorrected, thereby undesirablytilting the image surface in a direction away from an object.

For correcting lateral chromatic aberration more favorably in the lenssystem according to the present invention which has any one of the firstthrough fourth compositions, it is desirable to configure the radialtype gradient index lens component so as to satisfy the followingcondition (5):

    1/V.sub.1 <0.012                                           (5)

Though a radial type gradient index lens component which is configuredso as to satisfy the condition (1) produces chromatic aberration in anamount smaller than that of chromatic aberration produced by ahomogeneous lens component having a refractive power which is the sameas that of the radial type gradient index lens component, the radialtype gradient index lens component exhibits a lower chromatic aberrationcorrecting effect when V₁ has a value close to a value of V₀. It istherefore desirable that the radial type gradient index lens componentsatisfies the condition (5). If the condition (5) is not satisfied,lateral chromatic aberration will undesirably be undercorrected.

For obtaining a high performance image pickup lens system by using animage pickup device having picture elements arranged at a narrow pitchwith the lens system according to the present invention which has anyone of the compositions described above, it is desirable to configurethe radial type gradient index lens component so as to satisfy thefollowing condition (6):

    -0.2<1/V.sub.1 <0.007                                      (6)

If the upper limit of 0.007 of the condition (6) is exceeded, lateralchromatic aberration will undesirably be undercorrected in the lenssystem as a whole. If the lower limit of -0.2 of the condition (6) isnot satisfied, lateral chromatic aberration will undesirably beovercorrected in the lens system as a whole.

When the above-mentioned condition (1) is satisfied by the radial typegradient index lens component used in the lens system according to thepresent invention which has any one of the compositions described above,the radial type gradient index lens component produces chromaticaberration in an amount smaller than that of chromatic aberrationproduced by a homogeneous lens component having a refractive power whichis the same as that of the radial type gradient index lens component.When the lens system according to the present invention is to beconfigured so as to have a wide field angle, however, it is desirablefor favorably correcting lateral chromatic aberration to slightlyovercorrect chromatic aberration by a medium of the radial type gradientindex lens component. For this purpose, it is desirable to satisfy thefollowing condition (7):

    -0.1<1/V.sub.1 <0                                          (7)

When the condition (7) mentioned above is satisfied, a medium of theradial type gradient index lens component produces slightlyovercorrected lateral chromatic aberration, thereby making it possibleto correct lateral chromatic aberration favorably in the lens system asa whole. If the upper limit of 0 of the condition (7) is exceeded,lateral chromatic aberration will undesirably be undercorrected in thelens system as a whole. If the lower limit of -0.1 of the condition (7)is not satisfied, in contrast, lateral chromatic aberration willundesirably be overcorrected in the lens system as a whole.

For correcting lateral chromatic aberration or a Petzval's sum favorablyin the lens system according to the present invention which has any oneof the compositions described above, it is desirable to satisfy thecondition (2). When the lens system according to the present inventionis to be used in a highly precise fine image system which uses an imagepickup device having picture elements arranged at a narrow pitch,however, it is desirable to configure the radial type gradient indexlens component so as to satisfy the following condition (8):

    -0.4<N.sub.1 ×f.sup.2 <-0.05                         (8)

If the upper limit of -0.05 of the condition (8) is exceeded, lateralchromatic aberration and a Petzval's sum will undesirably beundercorrected. If the lower limit of -0.4 of the condition (8) is notsatisfied, in contrast, lateral chromatic aberration will undesirably beovercorrected.

Flare may be produced and transmittance may be lowered when the radialtype gradient index lens component is extremely thick in the lens systemaccording to the present invention which has any one of the compositionsdescribed above. Therefore, it is desirable that the radial typegradient index lens component used in the lens system according to thepresent invention which has any one of the compositions satisfies thefollowing condition (9):

    0.5<t.sub.G /f<10                                          (9)

wherein the reference symbol tG represents thickness of the radial typegradient index lens component.

If the lower limit of 0.5 of the condition (9) is not satisfied, theradial type gradient index lens component will have a weak refractivepower of medium and undesirably undercorrect lateral chromaticaberration. If the upper limit of 10 of the condition (9) is exceeded,in contrast, the radial type gradient index lens component will bethick, thereby undesirably producing flare and lowering transmittance.

Further, it is possible to control production amounts of sphericalaberration and coma by selecting an adequate value for the distributioncoefficient of the fourth order N₂ of the radial type gradient indexlens component to be used in the lens system according to the presentinvention which has any one of the compsitions. For favorably correctingcoma posing a problem in the lens system according to the presentinvention which has any one of the compsitions and a wide field angle inparticular, it is therefore desirable to configure the radial typegradient index lens component so as to satisfy the following condition(10):

    -0.5<N.sub.2 ×f.sup.4 <0.5                           (10)

If the lower limit of -0.5 of the condition (10) is not satisfied, comawill undesirably be undercorrected. If the upper limit of 0.5 of thecondition (10) is exceeded, in contrast, coma will undesirably beovercorrected.

When the lens system according to the present invention which has anyone of the compositions described above is to be applied to a systemrequiring higher imaging performance, it is desirable to satisfy, inplace of the condition (10), the following condition (11):

    -0.3<N.sub.2 ×f.sup.4 <0.3                           (11)

If the lower limit of -0.3 of the condition (11) is not satisfied, comawill be undercorrected or if the upper limit of 0.3 of the condition(11) is exceeded, in contrast, coma will be overcorrected, therebymaking it impossible to obtain high imaging performance in either case.

The lens system according to the present invention which has any one ofthe compositions described above is applicable not only to image pickupsystems and various types of measuring apparatuses but also to objectivelens systems for endoscopes.

Further, it is possible to obtain image pickup lens systems which arecompact and can be manufactured at low cost by composing the homogeneouslens component or the radial type gradient index lens component of anoptical element having an effect of a low pass filter or a band cutfilter cutting off components having specific wavelengths.

When a CCD which has sensitivity within the infrared wavelength region,for example, is to be used as an image pickup device, it is necessary todispose a cut filter which cuts off components having wavelengths withinthe infrared region in a lens system. When a radial type gradient indexlens component, for example, has a function to cut off the componentshaving wavelengths within the infrared region, it is unnecessary to usean additional infrared cut filter, thereby making it possible toconfigure the lens system to be more compact and lower the manufacturingcost thereof.

Furthermore, it is possible to correct aberrations more favorably in thelens system according to the present invention which has any one of thecompositions by configuring at least one of the surfaces of thehomogeneous lens component or the radial type gradient index lenscomponent as an aspherical surface. When a surface of the gradient indexlens component is to be configured as an aspherical surface, theaspherical surface can be formed by cementing or bringing a transparentmaterial such as resin, glass or liquid to or into close contact.Further, an aspherical surface can be formed on a radial type gradientindex lens component by precision grinding and polishing.

For correcting distortion and coma, in particular, favorably in the lenssystem according to the present invention which has any one of thecompositions, it is desirable to configure the radial type gradientindex lens component to be disposed on the object side of the stop so asto have a convex image side surface and a refractive index distributionin which refractive indices are lowered from the optical axis toward amargin. When the radial type gradient index lens component is configuredas described above, it has a refractive power of surface which isweakened from the optical axis toward the margin, thereby making itpossible to favorably correct distortion and coma produced by thissurface.

For correcting offaxial aberrations in particular, favorably in the lenssystem according to the present invention which has any one of thecompositions, it is desirable to use at least one meniscus lenscomponent having a concave surface on the stop side. It is desirable toimpart a negative refractive power to this meniscus lens component whenit is to be disposed in the first lens unit located on the object sideof the stop, or it is preferable to impart a positive refractive powerto the meniscus lens component when it is to be disposed in the secondlens unit located on the image side of the stop.

In the lens system according to the present invention which has a widefield angle, it is difficult to correct offaxial aberrations produced bythe negative lens component on the object side of the stop and offaxialaberrations produced by the positive lens components on the image sideof the stop. For correcting these offaxial aberrations, it is desirableto use a meniscus lens component as described above. A meniscus lenscomponent, one which has radii of curvature of small absolute values onboth surfaces in particular, may hardly be worked with high precision.However, a meniscus lens component, even if it has high curvature, caneasily be manufactured by cementing or bringing into close contactplanar surfaces of a concavo-planar lens element and a plano-convex lenselement which can be worked relatively easily.

For correcting lateral chromatic aberration favorably in the lens systemaccording to the present invention which has any one of the compositionsdescribed above, it is desirable to use at least one cemented lenscomponent in the lens system. Lateral chromatic aberration can becorrected more favorably particularly by disposing a cemented lenscomponent on the image side of the stop.

It is desirable to use a single radial type gradient index lenscomponent so that the lens system can be manufactured at a low cost.

Further, the lens system can be configured to be compact by disposing areflecting surface in the lens system.

For reducing the manufacturing cost of the lens system by lowering acost required for polishing the lens components, it is desirable toconfigure at least one lens component so as to have at least one planarsurface.

The lens system according to the present invention can be focused ontoan extremely short object distance by moving some of the lens componentsor the lens system as a whole along the optical axis.

When the first lens unit which is disposed on the object side of thestop is to be composed of at least one positive lens element and atleast one negative lens element in the lens system according to thepresent invention which has any one of the compositions described above,it is desirable for correcting lateral chromatic aberration produced bythe first lens unit to satisfy the following condition (12):

    ν.sub.p /ν.sub.n <0.95                               (12)

wherein the reference symbols ν_(p) and ν_(n) represent Abbe's numbersof the positive lens component and the negative lens componentrespectively disposed in the first lens unit.

If the condition (12) is not satisfied, lateral chromatic aberrationwill be overcorrected and cannot be corrected sufficiently in the lenssystem as a whole.

When the lens system according to the present invention which has anyone of the compositions described above is to be applied to a systemrequiring higher imaging performance, it is desirable to satisfy notonly the above-mentioned condition (10), but also the followingcondition (13):

    ν.sub.p /ν.sub.n <0.75                               (13)

If the condition (13) is not satisfied, lateral chromatic aberrationwill be undercorrected in the lens system, or correction of lateralchromatic aberration will undesirably be insufficient for application tothe system requiring the higher imaging performance. For correctingPetzval's sum in particular in the lens system according to the presentinvention which has any one of the compositions described above, it isdesirable to use, in the lens system, at least one positive lenscomponent and at least one negative lens component which satisfy thefollowing condition (14):

    1<N.sub.p /N.sub.n                                         (14)

wherein the reference symbol N_(p) represents a refractive index of thepositive lens component and the reference symbol N_(n) designates arefractive index of the negative lens component.

When the condition (14) is satisfied, a positive Petzval's sum can becorrected favorably. If the condition (14) is not satisfied, a Petzval'ssum will be undercorrected, thereby tilting the image surface toward theobject side.

When the lens system according to the present invention which has anyone of the compositions described above is to be configured so as tohave a relatively wide field angle, it is desirable for favorablecorrection of a Petzval's sum to satisfy, in place of the condition(14), the following condition (15):

    1.1<N.sub.p /N.sub.n                                       (15)

If the condition (15) is not satisfied, a Petzval's sum will beundercorrected.

When the first lens unit comprises at least one positive lens componentin the lens system according to the present invention which has any oneof the compositions described above, it is desirable for favorablecorrection of lateral chromatic aberration in the lens system as a wholeto select, for the positive lens component comprised in the first lensunit, a material having a dispersing power relatively high enough tosatisfy the following condition (16):

    ν.sub.p <50                                             (16)

wherein the reference symbol ν_(p) represents an Abbe's number of thepositive lens component comprised in the first lens unit.

When the condition (16) is satisfied, lateral chromatic aberration canbe corrected favorably in the lens system as a whole. If the condition(16) is not satisfied, lateral chromatic aberration will undesirably beundercorrected in the lens system as a whole.

For correcting lateral chromatic aberration more favorably, it isdesirable to satisfy not only the condition (16) but also the followingcondition (17):

    ν.sub.p <42                                             (17)

If the condition (17) is not satisfied, lateral chromatic aberrationwill undesirably be undercorrected in the lens system as a whole.

The present invention adopts the square formula (a) for approximatingrefractive index variations of a medium of a gradient index material.Gradient index materials made of media whose refractive index variationsare expressed by formulae other than the formula (a) are also applicableto the lens system according to the present invention by approximatingthe variations by the formula (a).

The lens system according to the present invention is not limited tothose defined by the claims, but may have compositions which aresubstantially defined by the claims.

Now, the embodiments of the lens system according to the presentinvention will be described in detail below.

The embodiments of the lens system according to the present inventionhave compositions illustrated in FIGS. 1 through 50 and numerical datawhich is listed below:

Embodiment 1

f=2.27 mm, F-number=2.8, image height=1.85 mm, 2m=79.00

    ______________________________________                                        r.sub.1  = 3.5530                                                                       d.sub.1  = 1.0000                                                                       n.sub.1  = 1.84666                                                                           ν.sub.1  = 23.78                        r.sub.2  = 7.2949                                                                       d.sub.2  = 0.1000                                                   r.sub.3  = 2.5193                                                                       d.sub.3  = 0.6844                                                                       n.sub.2  = 1.88300                                                                           ν.sub.2  = 40.78                        r.sub.4  = 0.7524                                                                       d.sub.4  = 0.8850                                                   r.sub.5  = ∞                                                                      d.sub.5  = 0.3000                                                   r.sub.6  = ∞ (stop)                                                               d.sub.6  = 0.0500                                                   r.sub.7  = -6.6704                                                                      d.sub.7  = 0.6000                                                                       n.sub.3  = 1.49700                                                                           ν.sub.3  = 81.61                        r.sub.8  = -1.2025                                                                      d.sub.8  = 0.1000                                                   r.sub.9  = ∞                                                                      d.sub.9  = 4.7630                                                                       n.sub.4  (gradient index lens)                            r.sub.10  = -3.1659                                                                     d.sub.10  = 0.1000                                                  r.sub.11  = ∞                                                                     d.sub.11  = 0.7500                                                                      n.sub.5  = 1.48749                                                                           ν.sub.5  = 70.21                        r.sub.12  = ∞                                                                     d.sub.12  = 1.1900                                                  r.sub.13  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.65000  -1.7533 × 10.sup.-2                                                                2.4961 × 10.sup.-3                                                               7.0121 × 10.sup.-5                  C-line 1.64567  -1.7575 × 10.sup.-2                                                                2.4961 × 10.sup.-3                                                               7.0121 × 10.sup.-5                  F-line 1.66011  -1.7436 × 10.sup.-2                                                                2.4961 × 10.sup.-3                                                               7.0121 × 10.sup.-5                  ______________________________________                                    

1/V₀ =2.22×10⁻², 1/V₁ =-0.80×10⁻², N₁ ×f² =-0.090

t_(G) =/f2.10, N₂ ×f⁴ =0.066, ν_(p) /ν_(n) =0.58

N_(P) /N_(n) =0.981, ν_(p) =23.78

Embodiment 2

f=2.86 mm, F-number=2.8, image height=1.85 mn, 2m=68.50

    ______________________________________                                        r.sub.1  = 5.7537                                                                       d.sub.1  = 0.8000                                                                       n.sub.1  = 1.84666                                                                           ν.sub.1  = 23.78                        r.sub.2  = 26.0340                                                                      d.sub.2  = 0.1000                                                   r.sub.3  = 1.8606                                                                       d.sub.3  = 0.7795                                                                       n.sub.2  = 1.76182                                                                           ν.sub.2  = 26.52                        r.sub.4  = 0.9682                                                                       d.sub.4  = 0.5595                                                   r.sub.5  = -4.2257                                                                      d.sub.5  = 4.1244                                                                       n.sub.3  (gradient index lens)                            r.sub.6  = -2.7820                                                                      d.sub.6  = 0.1000                                                   r.sub.7  = 5.4979                                                                       d.sub.7  = 0.9000                                                                       n.sub.4  = 1.61800                                                                           ν.sub.4  = 63.39                        r.sub.8  = -10.1657                                                                     d.sub.8  = 0.7424                                                   r.sub.9  = ∞                                                                      d.sub.9  = 0.7500                                                                       n.sub.5  = 1.48749                                                                           ν.sub.5  = 70.21                        r.sub.10  = ∞                                                                     d.sub.10  = 1.1900                                                  r.sub.11  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.65000  -1.5265 × 10.sup.-2                                                                2.0401 × 10.sup.-4                                                               1.3920 × 10.sup.-4                  C-line 1.64567  -1.5314 × 10.sup.-2                                                                2.0401 × 10.sup.-4                                                               1.3920 × 10.sup.-4                  F-line 1.66011  -1.5150 × 10.sup.-2                                                                2.0401 × 10.sup.-4                                                               1.3920 × 10.sup.-4                  ______________________________________                                    

(Focusing)

    ______________________________________                                        Object distance infinite                                                                        Object distance: 50 mm                                      ______________________________________                                        d.sub.2 0.1           0.9039                                                  ______________________________________                                    

1/V₀ =2.22×10⁻², 1/V₁ =-1.07×10⁻², N₁ ×f² =-0.125

t_(G) /f=1.44, N₂ ×f⁴ =0.014, ν_(p) /ν_(n) =0.90

N_(p) /N_(n) =1.048, ν_(p) =23.78

Embodiment 3

f=3.46 mm, F-number=2.8, image height=1.85 mm, 2ω=58.10

    ______________________________________                                        r.sub.1  = 6.6606                                                                       d.sub.1  = 1.0000                                                                       n.sub.1  = 1.84666                                                                           ν.sub.1  = 23.78                        r.sub.2  = -135.2532                                                                    d.sub.2  = 0.1000                                                   r.sub.3  = 43.7393                                                                      d.sub.3  = 0.7000                                                                       n.sub.2  = 1.81600                                                                           ν.sub.2  = 46.62                        r.sub.4  = 2.0326                                                                       d.sub.4  = 2.4197                                                   r.sub.5  = ∞ (stop)                                                               d.sub.5  = 1.3352                                                   r.sub.6  = ∞                                                                      d.sub.6  = 4.4721                                                                       n.sub.3  = (gradient index                                                    lens)                                                     r.sub.7  = -4.1696                                                                      d.sub.7  = 1.0000                                                   r.sub.8  = ∞                                                                      d.sub.8  = 1.6000                                                                       n.sub.4  = 1.51633                                                                           ν.sub.4  = 64.15                        r.sub.9  = ∞                                                                      d.sub.9  = 1.8180                                                                       n.sub.5  = 1.51633                                                                           ν.sub.5  = 64.15                        r.sub.10  = ∞                                                                     d.sub.10  = 1.6000                                                  r.sub.11  = ∞                                                                     d.sub.11  = 0.7500                                                                      n.sub.6  = 1.48749                                                                           ν.sub.6  = 70.21                        r.sub.12  = ∞                                                                     d.sub.12  = 1.1887                                                  r.sub.13  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.70000  -1.0460 × 10.sup.-2                                                                2.9333 × 10.sup.-4                                                               1.6591 × 10.sup.-5                  C-line 1.69533  -1.0614 × 10.sup.-2                                                                2.9333 × 10.sup.-4                                                               1.6591 × 10.sup.-5                  F-line 1.71089  -1.0102 × 10.sup.-2                                                                2.9333 × 10.sup.-4                                                               1.6591 × 10.sup.-5                  ______________________________________                                    

1/V₀ =2.22×1⁻², 1/V₁ =-4.89×10², N₁ ×f² =-0.125

t_(G) /f=1.29, N₂ ×f⁴ =0.042, ν_(p) /ν_(n) =0.51

N_(p) /N_(n) =1.017, ν_(p) =23.78

Embodiment 4

f=3.0 mm, F-number=2.8, image height=1.85 mm, 2ω=65.00

    ______________________________________                                        r.sub.1  = 7.5983                                                                       d.sub.1  = 1.0000                                                                       n.sub.1  = 1.84666                                                                           ν.sub.1  = 23.78                        r.sub.2  = 164.7782                                                                     d.sub.2  = 0.1000                                                   r.sub.3  = 17.0366                                                                      d.sub.3  = 0.7000                                                                       n.sub.2  = 1.78800                                                                           ν.sub.2  = 47.38                        r.sub.4  = 1.9781                                                                       d.sub.4  = 2.2073                                                   r.sub.5  = ∞ (stop)                                                               d.sub.5  = 1.1207                                                   r.sub.6  = ∞                                                                      d.sub.6  = 4.9793                                                                       n.sub.3  (gradient index lens)                            r.sub.7  = -3.8179                                                                      d.sub.7  = 1.0000                                                   r.sub.8  = ∞                                                                      d.sub.8  = 1.8180                                                                       n.sub.4  = 1.51633                                                                           ν.sub.4  = 64.15                        r.sub.9  = ∞                                                                      d.sub.9  = 1.6000                                                   r.sub.10  = ∞                                                                     d.sub.10  = 0.7500                                                                      n.sub.5  = 1.48749                                                                           ν.sub.5  = 70.21                        r.sub.11  = ∞                                                                     d.sub.11  = 1.1906                                                  r.sub.12  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.70000  -1.1339 × 10.sup.-2                                                                4.8463 × 10.sup.-4                                                               3.7447 × 10.sup.-5                  C-Line 1.69533  -1.1490 × 10.sup.-2                                                                4.8463 × 10.sup.-4                                                               3.7447 × 10.sup.-5                  F-Line 1.71089  -1.0988 × 10.sup.-2                                                                4.8463 × 10.sup.-4                                                               3.7447 × 10.sup.-5                  ______________________________________                                    

1/V₀ =2.22×10⁻², 1/V₁ =-4.43×10⁻², N₁ ×f² =-0.102

t_(G) /f=1.66, N₂ ×f⁴ =0.039, ν_(p) /ν_(f) =0.50

N_(p) /N_(n) =1.032, ν_(p) =23.78

Embodiment 5

f=2.39 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = 6.3059                                                                       d.sub.1  = 1.0000                                                                       n.sub.1  = 1.84666                                                                           ν.sub.1  = 23.78                        r.sub.2  = 18.6365                                                                      d.sub.2  = 0.1000                                                   r.sub.3  = 6.1423                                                                       d.sub.3  = 0.7000                                                                       n.sub.2  = 1.72916                                                                           ν.sub.2  = 54.68                        r.sub.4  = 1.4183                                                                       d.sub.4  = 1.5721                                                   r.sub.5  = ∞ (stop)                                                               d.sub.5  = 0.5021                                                   r.sub.6  = ∞                                                                      d.sub.6  = 4.1834                                                                       n.sub.3  (gradient index lens)                            r.sub.7  = -2.8211                                                                      d.sub.7  = 2.0376                                                   r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1933                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.70000  -2.0804 × 10.sup.-2                                                                1.5598 × 10.sup.-3                                                               2.7535 × 10.sup.-4                  C-line 1.69533  -2.1050 × 10.sup.-2                                                                1.5598 × 10.sup.-3                                                               2.7535 × 10.sup.-4                  F-line 1.71089  -2.0229 × 10.sup.-2                                                                1.5598 × 10.sup.-3                                                               2.7535 × 10.sup.-4                  ______________________________________                                    

(Focusing)

    ______________________________________                                        Object distance infinite                                                                        Object distance: 50 mm                                      ______________________________________                                        d.sub.4 1.5721        1.9210                                                  ______________________________________                                    

1/V₀ =2.22×10⁻², 1/V₁ =-3.95×10⁻², N₁ ×f² =-0.119

t_(G) /f=1.75, N₂ ×f⁴ =0.051, ν_(p) /ν_(n) =0.43

N_(p) /N_(n) =1.068, ν_(p) =23.78

Embodiment 6

f=2.64 mm, F-image=2.8, image height=1.85 min,

    ______________________________________                                        r.sub.1  = 4.9123                                                                       d.sub.1  = 1.0000                                                                       n.sub.1  = 1.84666                                                                           ν.sub.1  = 23.78                        r.sub.2  = 18.2873                                                                      d.sub.2  = 0.1000                                                   r.sub.3  = 4.6913                                                                       d.sub.3  = 0.7000                                                                       n.sub.2  = 1.77250                                                                           ν.sub.2  = 49.60                        r.sub.4  = 1.4914                                                                       d.sub.4  = 0.6000                                                   r.sub.5  = ∞                                                                      d.sub.5  = 0.1000                                                   r.sub.6  = -4.4328                                                                      d.sub.6  = 7.4209                                                                       n.sub.3  (gradient index lens)                            r.sub.7  = -2.7924                                                                      d.sub.7  = 1.9062                                                   r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1902                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

(Gradient index lens)

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.65000  -1.7286 × 10.sup.-2                                                                1.5861 × 10.sup.-3                                                               2.5037 × 10.sup.-4                  C-line 1.64567  -1.7331 × 10.sup.-2                                                                1.5861 × 10.sup.-3                                                               2.5037 × 10.sup.-4                  F-line 1.66011  -1.7181 × 10.sup.-2                                                                1.5861 × 10.sup.-3                                                               2.5037 × 10.sup.-4                  ______________________________________                                    

1/V₀ =2.22×10⁻², 1/V₁ =-0.89×10⁻², N₁ ×f² =-0.120

t_(g) /f=2.81, N₂ ×f⁴ =0.077, ν_(p) /ν_(n) =0.48

N_(p) /N_(n) =1.042, ν_(p) =23.78

Embodiment 7

f=296 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = 6.4539                                                                       d.sub.1  = 0.6000                                                                       n.sub.1  = 1.84666                                                                           ν.sub.1  = 23.78                        r.sub.2  = ∞                                                                      d.sub.2  = 0.0500                                                   r.sub.3  = 1.5195                                                                       d.sub.3  = 0.4000                                                                       n.sub.2  = 1.69680                                                                           ν.sub.2  = 55,53                        r.sub.4  = 1.0326                                                                       d.sub.5  = 0.4303                                                   r.sub.5  = ∞                                                                      d.sub.5  = 0.0200                                                   r.sub.6  = -2.8717                                                                      d.sub.6  = 4.8255                                                                       n.sub.3  (gradient index lens)                            r.sub.7  = -3.2719                                                                      d.sub.7  = 0.8283                                                   r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1901                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.70000  -3.8291 × 10.sup.-2                                                                -8.1824 × 10.sup.-5                                                              -1.5300 × 10.sup.-5                 C-line 1.69533  -3.8331 × 10.sup.-2                                                                -8.1824 × 10.sup.-5                                                              -1.5300 × 10.sup.-5                 F-line 1.71089  -3.8198 × 10.sup.-2                                                                -8.1824 × 10.sup.-5                                                              -1.5300 × 10.sup.-5                 ______________________________________                                    

1/V₀ =22.22×10⁻², 1/V₁ =0.35×10⁻², N₁ ×f² =-0.335

t_(G) /f=1.63, N₂ ×f⁴ =-0.006, ν_(p) /ν_(n) =0.43

N_(p) /N_(n) =1.088, ν_(p) =23.78

Embodiment 8

f=3.14 mm, F-number=2.0, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = -19.6459 (aspherical surface)                                                   d.sub.1 = 0.9000                                                                        n.sub.1 = 1.83400                                                                           ν.sub.1 = 37.17                        r.sub.2 = -6.7757                                                                         d.sub.2 = 0.0500                                                  r.sub.3 = 3.4584                                                                          d.sub.3 = 0.8000                                                                        n.sub.2 = 1.56907                                                                           ν.sub.2 = 71.30                        r.sub.4 = 1.6265                                                                          d.sub.4 = 1.2384                                                  r.sub.5 = ∞ (stop)                                                                  d.sub.5 = 1.2368                                                  r.sub.6 = ∞                                                                         d.sub.6 = 2.2372                                                                        n.sub.3 (gradient index lens)                           r.sub.7 = -2.9732                                                                         d.sub.7 = 1.6179                                                  r.sub.8 = ∞                                                                         d.sub.8 = 0.7500                                                                        n.sub.4 = 1.48749                                                                           ν.sub.4 = 70.21                        r.sub.9 = ∞                                                                         d.sub.9 = 1.1903                                                  r.sub.10 = ∞ (image)                                                    ______________________________________                                    

Aspherical surface coefficient

A₄ =-3.3410×10⁻³, A₆ =1.7516×10⁻⁴, A₆ =-8.3062×10⁻⁶

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.80000 -2.0124 × 10.sup.-2                                                                 1.1965 × 10.sup.-3                                                               3.4283 × 10.sup.-4                   C-line                                                                              1.79520 -2.0628 × 10.sup.-2                                                                 1.1965 × 10.sup.-3                                                               3.4283 × 10.sup.-4                   F-line                                                                              1.81120 -1.8950 × 10.sup.-2                                                                 1.1965 × 10.sup.-3                                                               3.4283 × 10.sup.-4                   ______________________________________                                    

1/V₀ =2.0×10⁻², 1/V₁ =-8.33×10⁻², N₁ ×f² =-0.198

t_(G) /f=0.71, N₂ ×f⁴ =0.116, ν_(p) /ν_(n) =0.52

N_(p) /N_(n) =1.169, ν_(p) =37.17

Embodiment 9

f=3.02 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = 4.2701                                                                          d.sub.1 = 0.8000                                                                        n.sub.1 = 1.78470                                                                           ν.sub.1 = 26.22                        r.sub.2 = 9.9483                                                                          d.sub.2 = 0.0500                                                  r.sub.3 = 2.5823                                                                          d.sub.3 = 0.6000                                                                        n.sub.2 = 1.72600                                                                           ν.sub.2 = 53.57                        r.sub.4 = 1.4322                                                                          d.sub.4 = 0.7000                                                  r.sub.5 = -22.0130                                                                        d.sub.5 = 5.4365                                                                        n.sub.3 (gradient index lens)                           r.sub.6 = -2.3784                                                                         d.sub.6 = 0.1200                                                                        n.sub.4 = 1.51742                                                                           ν.sub.4 = 52.42                        r.sub.7 = -2.3784 (aspherical surface)                                                    d.sub.7 = 0.4964                                                  r.sub.8 = ∞                                                                         d.sub.8 = 0.7500                                                                        n.sub.5 = 1.48749                                                                           ν.sub.5 = 70.21                        r.sub.9 = ∞                                                                         d.sub.9 = 1.1894                                                  r.sub.10 = ∞ (image)                                                    ______________________________________                                    

Aspherical surface coefficient

A₄ =2.4484×10⁻², A₆ =-5.5068×10⁻³, A₈ =2.1303×10⁻³

A₁₀ =-2.4346×10⁻⁴

Gradient index lens

    ______________________________________                                        N.sub.0        N.sub.1    N.sub.2                                             ______________________________________                                        d-line  1.76000    -1.7605 × 10.sup.-2                                                                -5.0256 × 10.sup.-5                       C-line  1.75562    -1.7605 × 10.sup.-2                                                                -5.0256 × 10.sup.-5                       F-line  1.77023    -1.7606 × 10.sup.-2                                                                -5.0256 × 10.sup.-5                       ______________________________________                                    

(Focusing)

    ______________________________________                                               object distance infinite                                                                  object distance 50 mm                                      ______________________________________                                        d.sub.8  0.4964        0.6684                                                 ______________________________________                                    

1/V₀ =1.92×10⁻², 1/V₁ =-0.64×10⁻⁴, N₁ ×f² =-0.161

t_(G) /f=1.80, N₂ ×f⁴ =-0.004, ν_(p) /ν_(n) =0.49

N_(p) /N_(n) =1.034, ν_(p) =26.22

Embodiment 10

f=3.07 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = -4.2073                                                                      d.sub.1 = 0.6000                                                                         n.sub.1 = 1.59551                                                                            ν.sub.1 = 39.21                         r.sub.2 = 1.8362                                                                       d.sub.2 = 0.3513                                                     r.sub.3 = 2.9430                                                                       d.sub.3 = 0.7000                                                                         n.sub.2 = 1.88300                                                                            ν.sub.2 = 40.78                         r.sub.4 = -3.4180                                                                      d.sub.4 = 0.5383                                                     r.sub.5 = ∞ (stop)                                                               d.sub.5 = 0.6832                                                     r.sub.6 = -2.5795                                                                      d.sub.6 = 5.0077                                                                         n.sub.3 (gradient index lens)                             r.sub.7 = -8.1779                                                                      d.sub.7 = 0.1000                                                     r.sub.8 = 8.8570                                                                       d.sub.8 = 0.8000                                                                         n.sub.4 = 1.69680                                                                            ν.sub.4 = 55.53                         r.sub.9 = ∞                                                                      d.sub.9 = 0.7500                                                                         n.sub.5 = 1.48749                                                                            ν.sub.5 = 70.21                         r.sub.10 = ∞                                                                     d.sub.10 = 1.1867                                                    r.sub.11 = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.65000 -2.7276 × 10.sup.-2                                                                 4.8438 × 10.sup.-4                                                               1.9531 × 10.sup.-4                   C-line                                                                              1.64567 -2.7317 × 10.sup.-2                                                                 4.8438 × 10.sup.-4                                                               1.9531 × 10.sup.-4                   F-line                                                                              1.66011 -2.7183 × 10.sup.-2                                                                 4.8438 × 10.sup.-4                                                               1.9531 × 10.sup.-4                   ______________________________________                                    

(Focusing)

    ______________________________________                                               object distance infinite                                                                  object distance 50 mm                                      ______________________________________                                        d.sub.2  0.3513        0.4332                                                 ______________________________________                                    

1/V₀ =2.22×10⁻², 1/V₁ =0.49×10⁻², N₁ ×f² =-0.069

t_(G) /f=1.63, N₂ ×f⁴ =0.043, ν_(p) /ν_(n) ×1.04

N_(p) /N_(n) =1.180, ν_(p) =40.78

Embodiment 11

f=3.07 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = -52.6988                                                                        d.sub.1 = 0.6000                                                                        n.sub.1 = 1.69680                                                                           ν.sub.1 = 55.53                        r.sub.2 = 2.0260                                                                          d.sub.2 = 1.2468                                                  r.sub.3 = 4.6995                                                                          d.sub.3 = 0.6000                                                                        n.sub.2 = 1.88300                                                                           ν.sub.2 = 40.78                        r.sub.4 = -3.9734                                                                         d.sub.4 = 0.2442                                                  r.sub.5 = ∞ (stop)                                                                  d.sub.5 = 0.3406                                                  r.sub.6 = -3.3187                                                                         d.sub.6 = 7.9754                                                                        n.sub.3 (gradient index lens)                           r.sub.7 = -11.9444                                                                        d.sub.7 = 0.1000                                                  r.sub.8 = ∞                                                                         d.sub.8 = 0.7500                                                                        n.sub.4 = 1.48749                                                                           ν.sub.4 = 70.21                        r.sub.9 = ∞                                                                         d.sub.9 = 1.1902                                                  r.sub.10 = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.65000 -1.8001 × 10.sup.-2                                                                 8.0872 × 10.sup.-4                                                               6.8852 × 10.sup.-5                   C-line                                                                              1.64567 -1.8047 × 10.sup.-2                                                                 8.0872 × 10.sup.-4                                                               6.8852 × 10.sup.-5                   F-line                                                                              1.66011 -1.7892 × 10.sup.-2                                                                 8.0872 × 10.sup.-4                                                               7.8852 × 10.sup.-5                   ______________________________________                                    

1/V₀ =2.22×10⁻², 1/V₁ =-0.86×10⁻², N₁ ×f² =-0.170

t_(G) /f=2.60, N₂ ×f⁴ =0.072, ν_(p) /ν_(n) =0.73

N_(p) /N_(n) =1.110, ν_(p) =40.78

Embodiment 12

f=2.59 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = 4.9859                                                                          d.sub.1 = 1.3500                                                                        n.sub.1 = 1.77250                                                                           ν.sub.1 = 49.60                        r.sub.2 = 2.7950                                                                          d.sub.2 = 0.4000                                                  r.sub.3 = 2.6177                                                                          d.sub.3 = 1.1000                                                                        n.sub.2 = 1.77250                                                                           ν.sub.2 = 49.60                        r.sub.4 = 1.2810                                                                          d.sub.4 = 0.9427                                                  r.sub.5 = ∞ (stop)                                                                  d.sub.5 = 0.2000                                                  r.sub.6 = 6.0933                                                                          d.sub.6 = 5.7745                                                                        n.sub.3 (gradient index lens)                           r.sub.7 = -2.8842                                                                         d.sub.7 = 1.8214                                                  r.sub.6 = ∞                                                                         d.sub.8 = 0.7500                                                                        n.sub.4 = 1.48749                                                                           ν.sub.4 = 70.21                        r.sub.9 = ∞                                                                         d.sub.9 = 1.1901                                                  r.sub.10 = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.70000  -1.4948 × 10.sup.-2                                                                1.5592 × 10.sup.-3                                                               4.2190 × 10.sup.-4                  C-line 1.69533  -1.5195 × 10.sup.-2                                                                1.5592 × 10.sup.-3                                                               4.2190 × 10.sup.-4                  F-line 1.71089  -1.4371 × 10.sup.-2                                                                1.5592 × 10.sup.-3                                                               4.2190 × 10.sup.-4                  ______________________________________                                    

1/V₀ =2.22×10⁻², 1/V₁ =-5.51×10⁻², N₁ ×f² =-0.100

t_(G) /f=2.23, N₂ ×f=0.070

Embodimenlt 13

f=3.64 mm, F-number=2.8, image height=1.85 mmn,

    ______________________________________                                        r.sub.1  = 2.0342                                                                       d.sub.1  = 0.8500                                                                       n.sub.1  = 1.80518                                                                           ν.sub.1  = 25.43                        r.sub.2  = 1.3259 (aspherical surface)                                                  d.sub.2  = 0.4500                                                   r.sub.3  = ∞ (stop)                                                               d.sub.3  = 0.2000                                                   r.sub.4  = -10.0164                                                                     d.sub.4  = 6.3071                                                                       n.sub.2  (gradient index lens)                            r.sub.5  = -3.6346                                                                      d.sub.5  = 0.1000                                                   r.sub.6  = -27.3485                                                                     d.sub.6  = 1.0000                                                                       n.sub.3  = 1.74100                                                                           ν.sub.3  = 52.65                        r.sub.7  = -9.9029                                                                      d.sub.7  = 2.3402                                                   r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1901                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

Aspherical surface coefficient

A₄ =-6.9685×10⁻², A2.7911×10⁻¹, A-3.3381×10⁻¹

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.75000  -9.5571 × 10.sup.-3                                                                5.7265 × 10.sup.-4                                                               1.6340 × 10.sup.-5                  C-line 1.74550  -9.7013 × 10.sup.-3                                                                5.7265 × 10.sup.-4                                                               1.6340 × 10.sup.-5                  F-line 1.76050  -9.2207 × 10.sup.-3                                                                5.7265 × 10.sup.-4                                                               1.6340 × 10.sup.-5                  ______________________________________                                    

1/V₀ =2.0×10⁻², 1/V₁ =-5.03×10⁻², N₁ ×f² =-0.127

t_(G) /f=1.73, N₂ ×f⁴ =0.100, N_(p) /N_(n) =0.964

Embodiment 14

f=3.82 mm, F-number=2.8, image height=1.85 mn,

    ______________________________________                                        r.sub.1  = 2.2577                                                                       d.sub.1  = 0.7500                                                                       n.sub.1  = 1.83400                                                                           ν.sub.1  = 37.17                        r.sub.2  = 1.5259                                                                       d.sub.2  = 0.4000                                                   r.sub.3  = ∞ (stop)                                                               d.sub.3  = 0.2525                                                   r.sub.4  = -11.0756                                                                     d.sub.4  = 5.8515                                                                       n.sub.2  (gradient index lens)                            r.sub.5  = -3.1807                                                                      d.sub.5  = 0.1000                                                   r.sub.6  = 5.8102                                                                       d.sub.6  = 1.4245                                                                       n.sub.3  = 1.83481                                                                           ν.sub.3  = 42.72                        r.sub.7  = 4.2325                                                                       d.sub.7  = 1.3322                                                   r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1903                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.65000  -1.3647 × 10.sup.-2                                                                3.7226 × 10.sup.-4                                                               8.7315 × 10.sup.-5                  C-line 1.64567  -1.3790 × 10.sup.-2                                                                3.7226 × 10.sup.-4                                                               8.7315 × 10.sup.-5                  F-line 1.66011  -1.3314 × 10.sup.-2                                                                3.7226 × 10.sup.-4                                                               8.7315 × 10.sup.-5                  ______________________________________                                    

1/V₀ =2.22×10⁻², 1/V₁ =3.49×10⁻², N₁ ×f² =10.99

t_(G) /f=1.53, N₂ ×f⁴ =0.079

Embodiment 15

f=3.57 mm, F-number=2.8, image height=1.85 mn,

    ______________________________________                                        r.sub.1  = 2.4980                                                                       d.sub.1  = 0.7800                                                                       n.sub.1  = 1.88300                                                                           ν.sub.1  = 40.78                        r.sub.2  = 1.5033                                                                       d.sub.2  = 0.7500                                                   r.sub.3  = ∞ (stop)                                                               d.sub.3  = 0.2000                                                   r.sub.4  = 3.2307                                                                       d.sub.4  = 0.6000                                                                       n.sub.2  = 1.76182                                                                           ν.sub.2  = 26.55                        r.sub.5  = 2.6645                                                                       d.sub.5  = 0.3000                                                   r.sub.6  = -38.9333                                                                     d.sub.6  = 4.4475                                                                       n.sub.3  (gradient index lens)                            r.sub.7  = -3.0418                                                                      d.sub.7  = 3.0516                                                   r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1900                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.68000  -1.7550 × 10.sup.-2                                                                2.1285 × 10.sup.-4                                                               3.5758 × 10.sup.-5                  C-line 1.67514  -1.7719 × 10.sup.-2                                                                2.1285 × 10.sup.-4                                                               3.5758 × 10.sup.-5                  F-line 1.69133  -1.7157 × 10.sup.-2                                                                2.1285 × 10.sup.-4                                                               3.5758 × 10.sup.-5                  ______________________________________                                    

1/V₀ =2.38×10⁻², 1/V₁ =-3.20×10⁻², N₁ ×f² =-0.224

t_(G) /f=1.25, N₂ ×f₄ =0.035

Embodiment 16

f=3.41 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = 1.7346                                                                       d.sub.1  = 0.7342                                                                       n.sub.1  = 1.81600                                                                           ν.sub.1  = 46.62                        r.sub.2  = 0.9887                                                                       d.sub.2  = 0.4000                                                   r.sub.3  = ∞ (stop)                                                               d.sub.3  = 0.2000                                                   r.sub.4  = -39.6075                                                                     d.sub.4  = 2.0388                                                                       n.sub.2  = 1.72916                                                                           ν.sub.2  = 54.68                        r.sub.5  = -1.8409                                                                      d.sub.5  = 0.3202                                                   r.sub.6  = -2.6460                                                                      d.sub.6  = 3.6440                                                                       n.sub.3  (gradient index lens)                            r.sub.7  = -3.7659                                                                      d.sub.7  = 1.7547                                                   r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1926                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.68000  -1.7780 × 10.sup.-2                                                                6.1714 × 10.sup.-4                                                               9.9867 × 10.sup.-5                  C-line 1.67514  -1.7942 × 10.sup.-2                                                                6.1714 × 10.sup.-4                                                               9.9867 × 10.sup.-5                  F-line 1.69133  -1.7403 × 10.sup.-2                                                                6.1714 × 10.sup.-4                                                               9.9867 × 10.sup.-5                  ______________________________________                                    

Embodiment 17

f=3.81 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = 10.6141                                                                      d.sub.1  = 0.8000                                                                       n.sub.1  = 1.56907                                                                           ν.sub.1  = 71.30                        r.sub.2  = 2.7054                                                                       d.sub.2  = 1.7762                                                   r.sub.3  = ∞ (stop)                                                               d.sub.3  = 1.3145                                                   r.sub.4  = ∞                                                                      d.sub.4  = 8.1554                                                                       n.sub.2  (gradient index lens)                            r.sub.5  = -5.5218                                                                      d.sub.5  = 1.0000                                                   r.sub.6  = ∞                                                                      d.sub.6  = 1.6000                                                                       n.sub.3  = 1.51633                                                                           ν.sub.3  = 64.15                        r.sub.7  = ∞                                                                      d.sub.7  = 2.0200                                                                       n.sub.4  = 1.51633                                                                           ν.sub.4  = 64.15                        r.sub.8  = ∞                                                                      d.sub.8  = 1.6000                                                   r.sub.9  = ∞                                                                      d.sub.9  = 0.7500                                                                       n.sub.5  = 1.48749                                                                           ν.sub.5  = 70.21                        r.sub.10  = ∞                                                                     d.sub.10  = 1.0009                                                  r.sub.11  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.70000 -7.3313 × 10.sup.-3                                                                 1.0613 × 10.sup.-4                                                               2.9899 × 10.sup.-6                   C-line                                                                              1.69533 -7.4031 × 10.sup.-3                                                                 1.0613 × 10.sup.-4                                                               2.9899 × 10.sup.-6                   F-line                                                                              1.71089 -7.1636 × 10.sup.-3                                                                 1.0613 × 10.sup.-4                                                               2.9899 × 10.sup.-6                   ______________________________________                                    

1/V₀ =2.22×10⁻², 1/V₁ =-3.27×10⁻², N₁ ×f² =-0.106

t_(G) /f=2.14, N₂ ×f⁴ =0.022

Embodimnent 18

f=3.51 mm, f-number=2.8, Image height=1.85 mm,

    ______________________________________                                        r.sub.1 = ∞ (stop)                                                               d.sub.1 = 0.1500                                                     r.sub.2 = -4.2775                                                                      d.sub.2 = 0.5000                                                                         n.sub.1 = 1.88300                                                                            ν.sub.1 = 40.78                         r.sub.3 = -2.2363                                                                      d.sub.3 = 0.3000                                                     r.sub.4 = -1.6050                                                                      d.sub.4 = 0.4000                                                                         n.sub.2 = 1.62004                                                                            ν.sub.2 = 36.26                         r.sub.5 = 2.5841                                                                       d.sub.5 = 1.0000                                                                         n.sub.3 = 1.88300                                                                            ν.sub.3 = 40.78                         r.sub.6 = -2.9650                                                                      d.sub.6 = 0.2500                                                     r.sub.7 = -2.2178                                                                      d.sub.7 = 3.9757                                                                         n.sub.4 (gradient index lens)                             r.sub.8 = -3.5973                                                                      d.sub.8 = 1.5185                                                     r.sub.9 = ∞                                                                      d.sub.9 = 0.7500                                                                         n.sub.5 = 1.48749                                                                            ν.sub.5 = 70.21                         r.sub.10 = ∞                                                                     d.sub.10 = 1.1900                                                    r.sub.11 = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.65000 -2.0714 × 10.sup.-2                                                                 9.1885 × 10.sup.-4                                                               1.6164 × 10.sup.-4                   C-line                                                                              1.64536 -2.0838 × 10.sup.-2                                                                 9.1885 × 10.sup.-4                                                               1.6164 × 10.sup.-4                   F-line                                                                              1.66083 -2.0424 × 10.sup.-2                                                                 9.1885 × 10.sup.-4                                                               1.6164 × 10.sup.-4                   ______________________________________                                    

1/V₀ =2.38×10⁻², 1/V₁ =-2.0×10⁻², N₁ ×f² =-0.255

t_(G) /f=1.13, N₂ ×f⁴ =0.139, N_(p) /N_(n) =1.162

Embodiment 19

f=3.34 mm, F-number=2.8, image height=1.85 m,

    ______________________________________                                        r.sub.1 = ∞ (stop)                                                                 d.sub.1 = 0.0500                                                   r.sub.2 = 8.3724                                                                         d.sub.2 = 0.6000                                                                        n.sub.1 = 1.77250                                                                           ν.sub.1 = 49.60                         r.sub.3 = -6.4998 (aspherical surface)                                                   d.sub.3 = 0.6004                                                   r.sub.4 = -1.3705                                                                        d.sub.4 = 3.0666                                                                        n.sub.2 (gradient index lens)                            r.sub.5 = -2.6175                                                                        d.sub.5 = 0.0500                                                   r.sub.6 = 4.5221                                                                         d.sub.6 = 0.4000                                                                        n.sub.3 = 1.84666                                                                           ν.sub.3 = 23.78                         r.sub.7 = 2.4448                                                                         d.sub.7 = 1.7896                                                                        n.sub.4 = 1.74100                                                                           ν.sub.4 = 52.65                         r.sub.8 = 8.1154                                                                         d.sub.8 = 0.5251                                                   r.sub.9 = ∞                                                                        d.sub.9 = 0.7500                                                                        n.sub.5 = 1.48749                                                                           ν.sub.5 = 70.21                         r.sub.10 = ∞                                                                       d.sub.10 = 1.1901                                                  r.sub.11 = ∞ (image)                                                    ______________________________________                                    

Aspherical surface coefficient

A₄ =-3.5905×10⁻², A₆ =1.6008×10⁻¹, A₄ =4.3928×10⁻¹

A₁₀ =4.0592×10⁻¹

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.68000 -1.8152 × 10.sup.-2                                                                 6.6614 × 10.sup.-4                                                               3.9599 × 10.sup.-4                   C-line                                                                              1.67566 -1.8152 × 10.sup.-2                                                                 6.6614 × 10.sup.-4                                                               3.9599 × 10.sup.-4                   F-line                                                                              1.69013 -1.8152 × 10.sup.-2                                                                 6.6614 × 10.sup.-4                                                               3.9599 × 10.sup.-4                   ______________________________________                                    

1/V₀ =2.13×10⁻², 1/V₁ =0, N₁ ×f² =-0.202

t_(G) /f=0.92, N₂ ×f⁴ =0.083, N_(p) /N_(n) =0.960

Embodiment 20

f=3.38 mm, f-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = ∞ (stop)                                                               d.sub.1 = 0.0500                                                     r.sub.2 = 36.0547                                                                      d.sub.2 = 0.6000                                                                         n.sub.1 = 1.88300                                                                            ν.sub.1 = 40.78                         r.sub.3 = -6.9767                                                                      d.sub.3 = 0.7937                                                     r.sub.4 = -1.6638                                                                      d.sub.4 = 2.7522                                                                         n.sub.2 (gradient index lens)                             r.sub.5 = -2.5548                                                                      d.sub.5 = 0.0500                                                     r.sub.6 = 4.7748                                                                       d.sub.6 = 2.1000                                                                         n.sub.3 = 1.77250                                                                            ν.sub.3 = 49.60                         r.sub.7 = -2.9710                                                                      d.sub.7 = 0.5000                                                                         n.sub.4 = 1.80518                                                                            ν.sub.4 = 25.43                         r.sub.9 = 7.8073                                                                       d.sub.8 = 0.3000                                                     r.sub.9 = ∞                                                                      d.sub.9 = 0.7500                                                                         n.sub.5 = 1.48749                                                                            ν.sub.5 = 70.21                         r.sub.10 = ∞                                                                     d.sub.10 = 1.1909                                                    r.sub.11 = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.66000 -1.5683 × 10.sup.-2                                                                 1.9657 × 10.sup.-4                                                               3.4070 × 10.sup.-4                   C-line                                                                              1.65479 -1.5699 × 10.sup.-2                                                                 1.9657 × 10.sup.-4                                                               3.4070 × 10.sup.-4                   F-line                                                                              1.67216 -1.5647 × 10.sup.-2                                                                 1.9657 × 10.sup.-4                                                               3.4070 × 10.sup.-4                   ______________________________________                                    

(Focusing)

    ______________________________________                                        Object distance infinite                                                                        Object distance 50 mm                                       ______________________________________                                        d.sub.8 0.3           0.5322                                                  ______________________________________                                    

1/V₀ =2.63×10⁻², 1/V₁ =3.33×10⁻², N₁ ×f² =-0.179

t_(G) /f=0.81, N₂ ×f⁴ =0.026, N_(p) /N_(n) =1.043

Embodiment 21

f=3.84 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = ∞ (stop)                                                               d.sub.1 = 0.2000                                                     r.sub.2 = 30.5251                                                                      d.sub.2 = 1.000                                                                          n.sub.1 = 1.88300                                                                            ν.sub.1 = 40.78                         r.sub.3 = -4.4735                                                                      d.sub.3 = 0.4950                                                     r.sub.4 = -2.4236                                                                      d.sub.4 = 4.7639                                                                         n.sub.2 = 1.88300                                                                            ν.sub.2 = 40.78                         r.sub.5 = -4.0027                                                                      d.sub.5 = 0.1500                                                     r.sub.6 = 6.5997                                                                       d.sub.6 = 2.4181                                                                         n.sub.3 (gradient index lens)                             r.sub.7 = 5.8898                                                                       d.sub.7 = 0.8000                                                     r.sub.8 = ∞                                                                      d.sub.8 = 0.7500                                                                         n.sub.4 = 1.48749                                                                            ν.sub.4 = 70.21                         r.sub.9 = ∞                                                                      d.sub.9 = 1.1897                                                     r.sub.10 = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.58000 -1.8284 × 10.sup.-2                                                                 2.1401 × 10.sup.-5                                                               6.3733 × 10.sup.-5                   C-line                                                                              1.57665 -1.8520 × 10.sup.-2                                                                 2.1401 × 10.sup.-5                                                               6.3733 × 10.sup.-5                   F-line                                                                              1.58781 -1.7735 × 10.sup.-2                                                                 2.1401 × 10.sup.-5                                                               6.3733 × 10.sup.-5                   ______________________________________                                    

1/V₀ =1.92×10⁻², 1/V₁ =4.29×10⁻², N₁ ×f² =-0.270

t_(G) /f=0.63, N₂ ×f⁴ =0.005

Embodiment 22

f=3.52 rmm, f-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = ∞ (stop)                                                               d.sub.1 = 0.1000                                                     r.sub.2 = -6.5217                                                                      d.sub.2 = 0.5000                                                                         n.sub.1 = 1.88300                                                                            ν.sub.1 = 40.78                         r.sub.3 = -3.3361                                                                      d.sub.3 = 0.9532                                                     r.sub.4 = -1.2433                                                                      d.sub.4 = 1.3987                                                                         n.sub.2 = 1.69895                                                                            ν.sub.2 = 30.12                         r.sub.5 = -1.9908                                                                      d.sub.5 = 0.1000                                                     r.sub.6 = -24.6308                                                                     d.sub.6 = 3.6086                                                                         n.sub.3 (gradient index lens)                             r.sub.7 = -4.6261                                                                      d.sub.7 = 1.8870                                                     r.sub.8 = ∞                                                                      d.sub.8 = 0.7500                                                                         n.sub.4 = 1.48749                                                                            ν.sub.4 = 70.21                         r.sub.9 = ∞                                                                      d.sub.9 = 1.1902                                                     r.sub.10 = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.65000  -1.4474 × 10.sup.-2                                                                3.6204 × 10.sup.-4                                                               2.8814 × 10.sup.-5                  C-line 1.64536  -1.4691 × 10.sup.-2                                                                3.6204 × 10.sup.-4                                                               2.8814 × 10.sup.-5                  F-line 1.66083  -3.3967 × 10.sup.-2                                                                3.6204 × 10.sup.-4                                                               2.8814 × 10.sup.-5                  ______________________________________                                    

1/V₀ =2.38×10⁻², 1/V₁ =-5.0×10⁻², N₁ ×f² =-0.179

t_(G) /f=1.03, N₂ ×f⁴ =0.056, N_(p) /N_(n) =1.108

Embodimient 23

f=3.94 mm, F-number=2.8, image height=1.85 min,

    ______________________________________                                        r.sub.1  = ∞ (stop)                                                               d.sub.1  = 0.1000                                                   r.sub.2  = 55.2262                                                                      d.sub.2  = 0.8000                                                                       n.sub.1  = 1.88300                                                                           ν.sub.1  = 40.78                        r.sub.3  = -2.2743                                                                      d.sub.3  = 0.3000                                                                       n.sub.2  = 1.48749                                                                           ν.sub.2  = 70.21                        r.sub.4  = -27.3072                                                                     d.sub.4  = 0.4000                                                   r.sub.5  = -1.6944                                                                      d.sub.5  = 4.4306                                                                       n.sub.3  (gradient index lens)                            r.sub.6  = -3.0741                                                                      d.sub.6  = 2.2066                                                   r.sub.7  = ∞                                                                      d.sub.7  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.8  = ∞                                                                      d.sub.8  = 1.1900                                                   r.sub.9  = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.65000  -2.0172 × 10.sup.-2                                                                6.9356 × 10.sup.-4                                                               1.4292 × 10.sup.-4                  C-line 1.64536  -2.0414 × 10.sup.-2                                                                6.9356 × 10.sup.-4                                                               1.4292 × 10.sup.-4                  F-line 1.66083  -1.9607 × 10.sup.-2                                                                6.9356 × 10.sup.-4                                                               1.4292 × 10.sup.-4                  ______________________________________                                    

1/V₀ =2.38×10⁻², 1/V₁ =4.0×10⁻², N₁ ×f² =-0.313

t_(G) /f=1.12, N₂ ×f⁴ =0.167, N_(p) /N_(n) =1.266

Embodimnent 24

f=3.54 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = ∞ (stop)                                                               d.sub.1  = 0.2500                                                   r.sub.2  = 21.7410                                                                      d.sub.2  = 1.1000                                                                       n.sub.1  = 1.72916                                                                           ν.sub.1  = 54.68                        r.sub.3  = -5.6428                                                                      d.sub.3  = 0.5703                                                   r.sub.4  = -1.7834                                                                      d.sub.4  = 4.2486                                                                       n.sub.2  (gradient index lens)                            r.sub.5  = -3.5184                                                                      d.sub.5  = 0.2000                                                   r.sub.6  = 5.0043                                                                       d.sub.6  = 3.0412                                                                       n.sub.3  = 1.61800                                                                           ν.sub.3  = 63.39                        r.sub.7  = 8.7136                                                                       d.sub.7  = 0.7000                                                   r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1899                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.80000  -1.0965 × 10.sup.-2                                                                1.2297 × 10.sup.-4                                                               6.2864 × 10.sup.-5                  C-line 1.79538  -1.1070 × 10.sup.-2                                                                1.2297 × 10.sup.-4                                                               6.2864 × 10.sup.-5                  F-line 1.81077  -1.0721 × 10.sup.-2                                                                1.2297 × 10.sup.-4                                                               6.2864 × 10.sup.-5                  ______________________________________                                    

1/V₀ =1.92×10⁻², 1/V₁ =3.18×10⁻², N₁ ×f² =-0.137

t_(G) /f=1.20, N₂ ×f⁴ =0.019

Embodiment 25

f=3.58 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = ∞ (stop)                                                               d.sub.1  = 0.2000                                                   r.sub.2  = ∞                                                                      d.sub.2  = 0.7500                                                                       n.sub.1  = 1.81600                                                                           ν.sub.1  = 46.62                        r.sub.3  = -3.9540                                                                      d.sub.3  = 0.4958                                                   r.sub.4  = -1.8736                                                                      d.sub.4  = 5.5165                                                                       n.sub.2  (gradient index lens)                            r.sub.5  = -3.6810                                                                      d.sub.5  = 0.1000                                                   r.sub.6  = 8.7934                                                                       d.sub.6  = 2.3198                                                                       n.sub.3  = 1.88300                                                                           ν.sub.3  = 40.78                        r.sub.7  = 5.9738                                                                       d.sub.7  = 0.7000                                                   r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1892                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.80000  -1.5871 × 10.sup.-2                                                                5.5386 × 10.sup.-4                                                               6.2203 × 10.sup.-5                  C-line 1.79400  -1.6027 × 10.sup.-2                                                                5.5386 × 10.sup.-4                                                               6.2203 × 10.sup.-5                  F-line 1.81400  -1.5506 × 10.sup.-2                                                                5.5386 × 10.sup.-4                                                               6.2203 × 10.sup.-5                  ______________________________________                                    

1/V₀ =2.5×10⁻², 1/V₁ =-3.28×10⁻², N₁ ×f² =-0.203

t_(G) /f=1.54, N₂ ×f⁴ =0.091, N_(p) /N_(n) =0.964

Embodiment 26

f=3.79 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = ∞ (stop)                                                               d.sub.1  = 0.2693                                                   r.sub.2  = -3.5315                                                                      d.sub.2  = 0.3500                                                                       n.sub.1  = 1.54814                                                                           ν.sub.1  = 45.78                        r.sub.3  = 2.3618                                                                       d.sub.3  = 0.8000                                                                       n.sub.2  = 1.88300                                                                           ν.sub.2  = 40.78                        r.sub.4  = -2.7122                                                                      d.sub.4  = 0.4000                                                   r.sub.5  = -1.6259                                                                      d.sub.5  = 5.2179                                                                       n.sub.3  (gradient index lens)                            r.sub.6  = -3.6461                                                                      d.sub.6  = 1.1191                                                   r.sub.7  = ∞                                                                      d.sub.7  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.8  = ∞                                                                      d.sub.8  = 1.1901                                                   r.sub.9  = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.65000  -1.8137 × 10.sup.-2                                                                8.6479 × 10.sup.-4                                                               6.6686 × 10.sup.-5                  C-line 1.64536  -1.8273 × 10.sup.-2                                                                8.6479 × 10.sup.-4                                                               6.6686 × 10.sup.-5                  F-line 1.66083  -1.7820 × 10.sup.-2                                                                8.6479 × 10.sup.-4                                                               6.6686 × 10.sup.-5                  ______________________________________                                    

1/V₀ =2.38×10², 1/V₁ =2.5×10⁻², N₁ ×f² =-0.261

t_(G) /f=1.38, N₂ ×f⁴ =0.178, N_(p) /N_(n) =1.216

Embodiment 27

f=4.0 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = ∞ (stop)                                                               d.sub.1  = 0.2000                                                   r.sub.2  = 3.4310                                                                       d.sub.2  = 0.7000                                                                       n.sub.1  = 1.56732                                                                           ν.sub.1  = 42.83                        r.sub.3  = 2.5037                                                                       d.sub.3  = 1.0000                                                   r.sub.4  = -1.8741                                                                      d.sub.4  = 0.9000                                                                       n.sub.2  = 1.83481                                                                           ν.sub.2  = 42.72                        r.sub.5  = -2.5404                                                                      d.sub.5  = 0.1000                                                   r.sub.6  = 17.9021                                                                      d.sub.6  = 5.7189                                                                       n.sub.3  (gradient index lens)                            r.sub.7  = -5.2222                                                                      d.sub.7  = 2.5960                                                   r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1878                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.70000  -1.0383 × 10.sup.-2                                                                6.8263 × 10.sup.-5                                                               4.6667 × 10.sup.-6                  C-line 1.69562  -1.0475 × 10.sup.-2                                                                6.8263 × 10.sup.-5                                                               4.7678 × 10.sup.-6                  F-line 1.71021  -1.0170 × 10.sup.-2                                                                6.8263 × 10.sup.-5                                                               4.7678 × 10.sup.-6                  ______________________________________                                    

1/V₀ =2.08×10⁻², 1/V₁ =-2.93×10⁻², N₁ ×f² =-0.166

t_(G) /f=1.43, N² ×f₄ =0.017

Embodiment 28

f=4.02 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = ∞ (stop)                                                               d.sub.1  = 0.2000                                                   r.sub.2  = 3.0004                                                                       d.sub.2  = 1.1000                                                                       n.sub.1  = 1.84666                                                                           ν.sub.1  = 23.78                        r.sub.3  = 1.9583                                                                       d.sub.3  = 0.5500                                                   r.sub.4  = -11.0859                                                                     d.sub.4  = 6.0725                                                                       n.sub.2  (gradient index lens)                            r.sub.5  = -4.3117                                                                      d.sub.5  = 0.2000                                                   r.sub.6  = 14.8211                                                                      d.sub.6  = 1.0000                                                                       n.sub.3  = 1.77250                                                                           ν.sub.3  = 49.60                        r.sub.7  = ∞                                                                      d.sub.7  = 2.1885                                                   r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1904                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.77000  -9.8448 × 10.sup.-3                                                                -5.8562 × 10.sup.-5                                                              -1.4079 × 10.sup.-5                 C-line 1.76422  -9.9688 × 10.sup.-3                                                                -5.8562 × 10.sup.-5                                                              -1.4079 × 10.sup.-5                 F-line 1.78347  -9.5555 × 10.sup.-3                                                                -5.8562 × 10.sup.-5                                                              -1.4079 × 10.sup.-5                 ______________________________________                                    

1/V₀ =2.5×10⁻², 1/V₁ =-4.20×10⁻², N₁ ×f² =-0.159

t_(G) /f=1.51, N₂ ×f⁴ =-0.015, N_(p) /N_(n) =0.960

Embodiment 29

f=3.67 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = ∞ (stop)                                                               d.sub.1  = 1.3500                                                   r.sub.2  = -1.6203                                                                      d.sub.2  = 1.0000                                                                       n.sub.1  = 1.60311                                                                           ν.sub.1  = 60.68                        r.sub.3  = -2.5622                                                                      d.sub.3  = 0.1000                                                   r.sub.4  = 17.5919                                                                      d.sub.4  = 5.7571                                                                       n.sub.2  (gradient index lens)                            r.sub.5  = -4.7537                                                                      d.sub.5  = 0.1000                                                   r.sub.6  = 11.3407                                                                      d.sub.6  = 2.0606                                                                       n.sub.3  = 1.80610                                                                           ν.sub.3  = 40.95                        r.sub.7  = 7.3687                                                                       d.sub.7  = 0.8000                                                   r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1887                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.68000  -1.0518 × 10.sup.-2                                                                2.1212 × 10.sup.-4                                                               1.6881 × 10.sup.-5                  C-line 1.67490  -1.0643 × 10.sup.-2                                                                2.1212 × 10.sup.-4                                                               1.6881 × 10.sup.-5                  F-line 1.69190  -1.0225 × 10.sup.-2                                                                2.1212 × 10.sup.-4                                                               1.6881 × 10.sup.-5                  ______________________________________                                    

1/V₀ =2.5×10⁻², 1/V₁ =3.98×10⁻², N₁ ×f² =-0.142

t_(G) /f=1.57, N₂ ×f⁴ =0.038

Embodiment 30

f=4.13 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = ∞ (stop)                                                               d.sub.1  = 1.8438                                                   r.sub.2  = -2.9770                                                                      d.sub.2  = 4.4968                                                                       n.sub.1  (gradient index lens)                            r.sub.3  = -3.9560                                                                      d.sub.3  = 0.1000                                                   r.sub.4  = 4.1810                                                                       d.sub.4  = 2.4922                                                                       n.sub.2  = 1.61800                                                                           ν.sub.2  = 63.39                        r.sub.5  = 4.3737                                                                       d.sub.5  = 0.7000                                                   r.sub.6  = 7.5621                                                                       d.sub.6  = 1.0000                                                                       n.sub.3  = 1.65160                                                                           ν.sub.3  = 58.52                        r.sub.7  = 9.3836                                                                       d.sub.7  = 0.5500                                                   r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1884                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.60000  -1.1209 × 10.sup.-2                                                                -1.1748 × 10.sup.-4                                                              2.4238 × 10.sup.-5                  C-line 1.59538  -1.1344 × 10.sup.-2                                                                -1.1748 × 10.sup.-4                                                              2.4238 × 10.sup.-5                  F-line 1.61077  -1.0895 × 10.sup.-2                                                                -1.1748 × 10.sup.-4                                                              2.4238 × 10.sup.-5                  ______________________________________                                    

1/V₀ =2.56×10⁻², 1/V₁ =4.0×10⁻², N₁ ×f² =0.191

t_(G) /f=1.09, N₂ ×f⁴ =-0.034

Embodiment 31

f=3.87 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = ∞ (stop)                                                               d.sub.1  = 1.0815                                                   r.sub.2  = -28.3102                                                                     d.sub.2  = 5.2277                                                                       n.sub.1  (gradient index lens)                            r.sub.3  = -3.5433                                                                      d.sub.3  = 6.0500                                                   r.sub.4  = -22.6176                                                                     d.sub.4  = 0.9500                                                                       n.sub.2  = 1.88300                                                                           ν.sub.2  = 40.78                        r.sub.5  = -4.0675                                                                      d.sub.5  = 0.5000                                                                       n.sub.3  = 1.62588                                                                           ν.sub.3  = 35.70                        r.sub.6  = 5.4289                                                                       d.sub.6  = 0.5000                                                   r.sub.7  = ∞                                                                      d.sub.7  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.8  = ∞                                                                      d.sub.8  = 1.1942                                                   r.sub.9  = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.75000  -1.6544 × 10.sup.-2                                                                1.1161 × 10.sup.-3                                                               8.7929 × 10.sup.-5                  C-line 1.74392  -1.6742 × 10.sup.-2                                                                1.1161 × 10.sup.-3                                                               8.7929 × 10.sup.-5                  F-line 1.76419  -1.6081 × 10.sup.-2                                                                1.1161 × 10.sup.-3                                                               8.7929 × 10.sup.-5                  ______________________________________                                    

1/V₀ =2×10⁻², 1/V₁ =-4.0×10⁻², N₁ ×f² =-0.248

t_(G) /f=1.35, N₂ ×f⁴ =0.250, N_(p) /N_(n) =1.158

Embodiment 32

f=53.88 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = ∞ (stop)                                                               d.sub.1  = 1.3240                                                   r.sub.2  = -34.3162                                                                     d.sub.2  = 5.1194                                                                       n.sub.1  (gradient index lens)                            r.sub.3  = -3.3533                                                                      d.sub.3  = 0.0500                                                   r.sub.4  = -15.7193                                                                     d.sub.4  = 0.4000                                                                       n.sub.2  = 1.53172                                                                           ν.sub.2  = 48.91                        r.sub.5  = 3.0343                                                                       d.sub.5  = 1.0000                                                                       n.sub.3  = 1.88300                                                                           ν.sub.3  = 40.78                        r.sub.6  = 4.4906 (aspherical surface)                                                  d.sub.6  = 0.5000                                                   r.sub.7  = ∞                                                                      d.sub.7  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.8  = ∞                                                                      d.sub.8  = 1.1823                                                   r.sub.9  = ∞ (image)                                                    ______________________________________                                    

Aspherical surface coefficient

A₄ =1.0615×10⁻², A₆ =-4.7062×10⁻³, A₆ =1.3538×10⁻³

A₁₀ ---1.7029×10⁻⁴

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.74000 -1.4577 × 10.sup.-2                                                                 4.4708 × 10.sup.-4                                                               2.1341 × 10.sup.-4                   C-line                                                                              1.73366 -1.4796 × 10.sup.-2                                                                 4.4708 × 10.sup.-4                                                               2.1341 × 10.sup.-4                   F-line                                                                              1.75480 -1.4067 × 10.sup.-2                                                                 4.4708 × 10.sup.-4                                                               2.1341 × 10.sup.-4                   ______________________________________                                    

1/V₀ =2.86×10⁻², 1/V₁ =5.0×10⁻², N₁ ×f² =-0.219

t_(g) /f=1.32, N₂ ×f⁴ =0.101, N_(p) /N_(n) =1.229

Embodiment 33

f=4.24 mm, f-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = ∞ (stop)                                                               d.sub.1 = 0.2000                                                     r.sub.2 = -6.1182                                                                      d.sub.2 = 8.0353                                                                         n.sub.1 (gradient index lens)                             r.sub.3 = -3.9652                                                                      d.sub.3 = 0.1000                                                     r.sub.4 = 7.9186                                                                       d.sub.4 = 1.0000                                                                         n.sub.2 = 1.84666                                                                            ν.sub.2 = 23.78                         r.sub.5 = 5.6744                                                                       d.sub.5 = 0.3000                                                     r.sub.6 = 6.7751                                                                       d.sub.6 = 0.9000                                                                         n.sub.3 = 1.88300                                                                            ν.sub.3 = 40.78                         r.sub.7 = 5.0200                                                                       d.sub.7 = 0.6000                                                     r.sub.8 = ∞                                                                      d.sub.8 = 0.7500                                                                         n.sub.4 = 1.48749                                                                            ν.sub.4 = 70.21                         r.sub.9 = ∞                                                                      d.sub.9 = 1.1827                                                     r.sub.10 = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.60000 -1.1573 × 10.sup.-2                                                                 2.1870 × 10.sup.-4                                                               4.1983 × 10.sup.-5                   C-line                                                                              1.59625 -1.1560 × 10.sup.-2                                                                 2.1870 × 10.sup.-4                                                               4.1983 × 10.sup.-5                   F-line                                                                              1.60875 -1.1605 × 10.sup.-2                                                                 2.1870 × 10.sup.-4                                                               4.1983 × 10.sup.-5                   ______________________________________                                    

1/V₀ =2.08×10⁻², 1/V₁ =0.39×10⁻², N₁ ×f² =-0.208

t_(G) /f=1.90, N₂ ×f⁴ =0.071

Embodiment 34

f=4.41 mm, f-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = ∞ (stop)                                                               d.sub.1 = 0.1000                                                     r.sub.2 = 2.5363                                                                       d.sub.2 = 0.6000                                                                         n.sub.1 = 1.88300                                                                            ν.sub.1 = 40.78                         r.sub.3 = 1.7360                                                                       d.sub.3 = 0.5000                                                     r.sub.4 = -9.9470                                                                      d.sub.4 = 6.2667                                                                         n.sub.2 (gradient index lens)                             r.sub.5 = -3.7009                                                                      d.sub.5 = 3.8629                                                     r.sub.6 = ∞                                                                      d.sub.6 = 0.7500                                                                         n.sub.3 = 1.48749                                                                            ν.sub.3 = 70.21                         r.sub.7 = ∞                                                                      d.sub.7 = 1.1897                                                     r.sub.8 = ∞ (image)                                                     ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.65000 -1.1811 × 10.sup.-2                                                                 4.3851 × 10.sup.-5                                                               5.0237 × 10.sup.-6                   C-line                                                                              1.64536 -1.1919 × 10.sup.-2                                                                 4.3851 × 10.sup.-5                                                               5.0237 × 10.sup.-6                   F-line                                                                              1.66083 -1.1558 × 10.sup.-2                                                                 4.3851 × 10.sup.-5                                                               5.0237 × 10.sup.-6                   ______________________________________                                    

1/V₀ =2.38×10⁻², 1/V₁ =-3.06×10⁻², N₁ ×f² =-0.230

t_(G) /f=1.42, N₂ ×f⁴ =0.017

Embodiment 35

f=4.11 mm, F-nuumer=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = ∞ (stop)                                                               d.sub.1 = 0.0500                                                     r.sub.2 = -10.4215                                                                     d.sub.2 = 0.7000                                                                         n.sub.1 = 1.71999                                                                            ν.sub.1 = 50.25                         r.sub.3 = -3.8884                                                                      d.sub.3 = 0.6652                                                     r.sub.4 = -2.3624                                                                      d.sub.4 = 10.2352                                                                        n.sub.2 (gradient index lens)                             r.sub.5 = 17.3389                                                                      d.sub.5 = 0.5000                                                     r.sub.6 = ∞                                                                      d.sub.6 = 0.7500                                                                         n.sub.3 = 1.48749                                                                            ν.sub.3 = 70.21                         r.sub.7 = ∞                                                                      d.sub.7 = 1.1901                                                     r.sub.8 = ∞ (image)                                                     ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.70000 -2.1513 × 10.sup.-2                                                                 1.8930 × 10.sup.-4                                                               5.9305 × 10.sup.-7                   C-line                                                                              1.69500 -2.1465 × 10.sup.-2                                                                 1.8930 × 10.sup.-4                                                               5.9305 × 10.sup.-7                   F-line                                                                              1.71167 -2.1625 × 10.sup.-2                                                                 1.8930 × 10.sup.-4                                                               5.9305 × 10.sup.-7                   ______________________________________                                    

1/V₀ =2.38×10⁻², 1/V₁ =0.74×10⁻², N₁ ×f² =-0.363

t_(g) /f=2.50, N₂ ×f⁴ =0.054

Embodiment 36

f=4.01 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = ∞ (stop)                                                               d.sub.1 = 0.2500                                                     r.sub.2 = -5.9959                                                                      d.sub.2 = 8.4514                                                                         n.sub.1 (gradient index lens)                             r.sub.3 = -4.0188                                                                      d.sub.3 = 0.1000                                                     r.sub.4 = -37.6735                                                                     d.sub.4 = 0.8000                                                                         n.sub.2 = 1.63980                                                                            ν.sub.2 = 34.48                         r.sub.5 = 4.4714                                                                       d.sub.5 = 0.7000                                                     r.sub.6 = ∞                                                                      d.sub.6 = 0.7500                                                                         n.sub.3 = 1.48749                                                                            ν.sub.3 = 70.21                         r.sub.7 = ∞                                                                      d.sub.7 = 1.1885                                                     r.sub.8 = ∞ (image)                                                     ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.75000 -1.8608 × 10.sup.-2                                                                 5.6704 × 10.sup.-4                                                               4.6884 × 10.sup.-5                   C-line                                                                              1.74550 -1.8607 × 10.sup.-2                                                                 5.6704 × 10.sup.-4                                                               4.6884 × 10.sup.-5                   F-line                                                                              1.76050 -1.8611 × 10.sup.-2                                                                 5.6704 × 10.sup.-4                                                               4.6884 × 10.sup.-5                   ______________________________________                                    

1/V₀ =2.0×10⁻², 1/V₁ =0.002×10⁻², N₁ ×f² =-0.299

t_(g) /f=2.11, N₂ ×f⁴ =0.145

Embodiment 37

f=4.37 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = ∞ (stop)                                                               d.sub.1 = 0.2000                                                     r.sub.2 = -4.1250                                                                      d.sub.2 = 4.8867                                                                         n.sub.1 (gradient index lens)                             r.sub.3 = -8.7362                                                                      d.sub.3 = 0.6500                                                     r.sub.4 = -5.4661                                                                      d.sub.4 = 1.0000                                                                         n.sub.2 = 1.81600                                                                            ν.sub.2 = 46.62                         r.sub.5 = -3.5559                                                                      d.sub.5 = 3.3703                                                     r.sub.6 = ∞                                                                      d.sub.6 = 0.7500                                                                         n.sub.3 = 1.48749                                                                            ν.sub.3 = 70.21                         r.sub.7 = ∞                                                                      d.sub.7 = 1.1884                                                     r.sub.8 = ∞ (image)                                                     ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.75000 -1.8462 × 10.sup.-2                                                                 9.1003 × 10.sup.-4                                                               1.5212 × 10.sup.-4                   C-line                                                                              1.74550 -1.8545 × 10.sup.-2                                                                 9.1003 × 10.sup.-4                                                               1.5212 × 10.sup.-4                   F-line                                                                              1.76050 -1.8269 × 10.sup.-2                                                                 9.1003 × 10.sup.-4                                                               1.5212 × 10.sup.-4                   ______________________________________                                    

1/V₀ =2.0×10⁻², 1/V₁ =1.50×10⁻², N₁ ×f² =-0.353

t_(G) /f=1.12, N₂ ×f⁴ =0.332

Embodiment 38

f=3.48 mm, f-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = ∞                                                                      d.sub.1 = 0.6000                                                                         n.sub.1 = 1.88300                                                                            ν.sub.1 = 40.78                         r.sub.2 = -7.3094                                                                      d.sub.2 = 0.0500                                                     r.sub.3 = ∞ (stop)                                                               d.sub.3 = 1.2030                                                     r.sub.4 = -1.5102                                                                      d.sub.4 = 2.6779                                                                         n2 (gradient index lens)                                  r.sub.5 = -2.4601                                                                      d.sub.5 = 0.0500                                                     r.sub.6 = 4.3486                                                                       d.sub.6 = 2.3000                                                                         n.sub.3 = 1.73400                                                                            ν.sub.3 = 51.49                         r.sub.7 = -2.9908                                                                      d.sub.7 = 0.5000                                                                         n.sub.4 = 1.78470                                                                            ν.sub.4 = 26.22                         r.sub.8 = 7.9176                                                                       d.sub.8 = 0.4000                                                     r.sub.9 = ∞                                                                      d.sub.9 = 0.7500                                                                         n.sub.5 = 1.48749                                                                            ν.sub.5 = 70.21                         r.sub.10 = ∞                                                                     d.sub.10 = 1.1833                                                    r.sub.11 = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.66000 -1.7096 × 10.sup.-2                                                                 5.9455 × 10.sup.-5                                                               3.4671 × 10.sup.-4                   C-line                                                                              1.65479 -1.7113 × 10.sup.-2                                                                 5.9455 × 10.sup.-5                                                               3.4671 × 10.sup.-4                   F-line                                                                              1.67216 -1.7056 × 10.sup.-2                                                                 5.9455 × 10.sup.-5                                                               3.4571 × 10.sup.-4                   ______________________________________                                    

(Focusing)

    ______________________________________                                        Object distance infinite                                                                        Object distance 200 mm                                      ______________________________________                                        d.sub.2 0.05          0.4068                                                  ______________________________________                                    

1/V₀ =2.63×10⁻², 1/V₁ =0.33×10⁻², N₁ ×f² =-0.207

t_(G) /f=0.77, N₂ ×f⁴ =0.009, N_(p) /N_(n) =1.055, ν_(p) =40.78

Embodiment 39

f=3.15 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = 9.0969                                                                       d.sub.1 = 0.7000                                                                         n.sub.1 = 1.84666                                                                            ν.sub.1 = 23.78                         r.sub.2 = -30.0731                                                                     d.sub.2 = 1.0005                                                     r.sub.3 = ∞ (stop)                                                               d.sub.3 = 0.5934                                                     r.sub.4 = -1.4531                                                                      d.sub.4 = 2.4469                                                                         n.sub.2 (gradient index lens)                             r.sub.5 = -2.3769                                                                      d.sub.5 = 0.0500                                                     r.sub.6 = 3.1021 (aspherical surface)                                                  d.sub.6 = 1.5652                                                                         n.sub.3 = 1.69680                                                                            ν.sub.3 = 55.53                         r.sub.7 = 8.8986                                                                       d.sub.7 = 0.5000                                                     r.sub.8 = ∞                                                                      d.sub.8 = 0.7500                                                                         n.sub.4 = 1.48749                                                                            ν.sub.4 = 70.21                         r.sub.9 = ∞                                                                      d.sub.9 = 1.1901                                                     r.sub.10 = ∞ (image)                                                    ______________________________________                                    

Aspherical surface coefficient

A₄ =1.0406×10³, A₆ =1.1767×10⁻³, A₆ =1.6092×10⁻⁴

A₁₀ =-1.2601×10⁻⁵

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.64000 -1.7068 × 10.sup.-2                                                                 -8.2755 × 10.sup.-5                                                              -2.6214 × 10.sup.-5                  C-line                                                                              1.63543 -1.7273 × 10.sup.-2                                                                 -8.2755 × 10.sup.-5                                                              -2.6214 × 10.sup.-5                  F-line                                                                              1.65067 -1.6590 × 10.sup.-2                                                                 -8.2755 × 10.sup.-5                                                              -2.6214 × 10.sup.-5                  ______________________________________                                    

1/V₀ =2.38×10⁻², 1/V₁ =-4.0×10⁻², N₁ ×f² =-0.169

t_(G) /f=0.78, N₂ ×f⁴ =-0.008, ν_(p) =23.78

Embodiment 40

f=3.5 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = -21.4456                                                                     d.sub.1 = 0.8000                                                                         n.sub.1 = 1.84666                                                                            ν.sub.1 = 23.78                         r.sub.2 = -4.9066                                                                      d.sub.2 = 0.5000                                                     r.sub.3 = ∞ (stop)                                                               d.sub.3 = 0.5199                                                     r.sub.4 = -1.7092                                                                      d.sub.4 = 5.2494                                                                         n.sub.2 (gradient index lens)                             r.sub.5 = -3.0713                                                                      d.sub.5 = 0.0500                                                     r.sub.6 = 7.3000                                                                       d.sub.6 = 2.3809                                                                         n.sub.3 = 1.88300                                                                            ν.sub.3 = 40.78                         r.sub.7 = 4.9758                                                                       d.sub.7 = 0.6000                                                     r.sub.8 = ∞                                                                      d.sub.8 = 0.7500                                                                         n.sub.4 = 1.48749                                                                            ν.sub.4 = 70.21                         r.sub.9 = ∞                                                                      d.sub.9 = 1.1897                                                     r.sub.10 = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.64000 -1.8357 × 10.sup.-2                                                                 5.0858 × 10.sup.-4                                                               1.7243 × 10.sup.-4                   C-line                                                                              1.63543 -1.8578 × 10.sup.-2                                                                 5.0858 × 10.sup.-4                                                               1.7243 × 10.sup.-4                   F-line                                                                              1.65067 -1.7843 × 10.sup.-2                                                                 5.0858 × 10.sup.-4                                                               1.7243 × 10.sup.-4                   ______________________________________                                    

1/V₀ =2.38×10⁻², 1/V₁ =-4.0×10⁻², N₁ ×f² =-0.225

t_(G) /f=1.50, N₂ ×f⁴ =-0.706, N_(p) /N_(n) =0.981, ν_(p) =23.78

Embodiment 41

f=3.48 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = -5.9937                                                                      d.sub.1 = 0.6000                                                                         n.sub.1 = 1.84666                                                                            ν.sub.1 = 23.78                         r.sub.2 = -3.4547                                                                      d.sub.2 = 0.2000                                                     r.sub.3 = ∞ (stop)                                                               d.sub.3 = 0.3000                                                     r.sub.4 = -2.1601                                                                      d.sub.4 = 0.4000                                                                         n.sub.2 = 1.45600                                                                            ν.sub.2 = 90.31                         r.sub.5 = 6.4339                                                                       d.sub.5 = 0.2000                                                     r.sub.6 = 5.3428                                                                       d.sub.6 = 4.5554                                                                         n.sub.3 (gradient index lens)                             r.sub.7 = -3.0070                                                                      d.sub.7 = 1.7836                                                     r.sub.8 = ∞                                                                      d.sub.8 = 0.7500                                                                         n.sub.4 = 1.48749                                                                            ν.sub.4 = 70.21                         r.sub.9 = ∞                                                                      d.sub.9 = 1.1900                                                     r.sub.10 = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.64000 -1.1984 × 10.sup.-2                                                                 8.5081 × 10.sup.-4                                                               3.1225 × 10.sup.-4                   C-line                                                                              1.63543 -1.2356 × 10.sup.-2                                                                 8.5081 × 10.sup.-4                                                               3.1225 × 10.sup.-4                   F-line                                                                              1.65067 -1.1117 × 10.sup.-2                                                                 8.5081 × 10.sup.-4                                                               3.1225 × 10.sup.-4                   ______________________________________                                    

1/V₀ =2.38×10⁻², 1/V₁ =10.03×10⁻², N₁ ×f² =-0.145

t_(G) /f=1.31, N₂ ×f⁴ =0.125, N_(p) /N_(n) =1.268, ν=23.78

Embodiment 42

f=3.2 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = 25.4222                                                                      d.sub.1 = 0.8000                                                                         n.sub.1 = 1.78470                                                                            ν.sub.1 = 26.30                         r.sub.2 = -14.6679                                                                     d.sub.2 = 1.4043                                                     r.sub.3 = ∞ (stop)                                                               d.sub.3 = 0.3674                                                     r.sub.4 = -1.9719                                                                      d.sub.4 = 3.5687                                                                         n.sub.2 = 1.83481                                                                            ν.sub.2 = 42.72                         r.sub.5 = -3.0383                                                                      d.sub.5 = 0.1500                                                     r.sub.6 = 3.6203                                                                       d.sub.6 = 2.4626                                                                         n.sub.3 (gradient index lens)                             r.sub.7 = 4.9906                                                                       d.sub.7 = 0.8000                                                     r.sub.8 = ∞                                                                      d.sub.8 = 0.7500                                                                         n.sub.4 = 1.48749                                                                            ν.sub.4 = 70.21                         r.sub.9 = ∞                                                                      d.sub.9 = 1.1901                                                     r.sub.10 = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0   N.sub.1     N.sub.2    N.sub.3                                      ______________________________________                                        d-line                                                                              1.58000 -1.5694 × 10.sup.-2                                                                 -3.3988 × 10.sup.-4                                                              4.2259 × 10.sup.-5                   C-line                                                                              1.57665 -1.5882 × 10.sup.-2                                                                 -3.3988 × 10.sup.-4                                                              4.2259 × 10.sup.-5                   F-line                                                                              1.58781 -1.5257 × 10.sup.-2                                                                 -3.3988 × 10.sup.-4                                                              4.2259 × 10.sup.-5                   ______________________________________                                    

1/V₀ =1.92×10⁻², 1/V₁ =-3.98×10⁻², N₁ ×f² =-0.161

t_(G) /f=0.77, N₂ ×f⁴ =-0.036, ν_(p=) 26.3

Embodiment 43

f=3.85 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = -7.9783                                                                      d.sub.1 = 0.8000                                                                         n.sub.1 = 1.74100                                                                            ν.sub.1 = 52.65                         r.sub.2 = -3.5855                                                                      d.sub.2 = 0.4932                                                     r.sub.3 = ∞ (stop)                                                               d.sub.3 = 0.4000                                                     r.sub.4 = -1.7848                                                                      d.sub.4 = 7.7391                                                                         n.sub.2 (gradient index lens)                             r.sub.5 = -3.9369                                                                      d.sub.5 = 3.1567                                                     r.sub.6 = ∞                                                                      d.sub.6 = 0.7500                                                                         n.sub.3 = 1.48749                                                                            ν.sub.3 = 70.21                         r.sub.7 = ∞                                                                      d.sub.7 = 1.1906                                                     r.sub.8 = ∞ (image)                                                     ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.60000  -1.3899 × 10.sup.-2                                                                2.1744 × 10.sup.-4                                                               3.2896 × 10.sup.-5                  C-line 1.59617  -1.3959 × 10.sup.-2                                                                2.1744 × 10.sup.-4                                                               3.2896 × 10.sup.-5                  F-line 1.60894  -1.3760 × 10.sup.-2                                                                2.1744 × 10.sup.-4                                                               3.2896 × 10.sup.-5                  ______________________________________                                    

1/V₀ =2.13×10⁻², 1/V₁ =-1343×10², N₁ ×f² =-0.206

t_(G) /f=2.01, N₂ ×f⁴ =0.048, ν_(p) =52.65

Embodiment 44

f=3.61 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = -3.4861                                                                      d.sub.1  = 0.8000                                                                       n.sub.1  = 1.77250                                                                           ν.sub.1  = 49.60                        r.sub.2  = -3.3510                                                                      d.sub.2  = 0.1500                                                   r.sub.3  = 58.9888                                                                      d.sub.3  = 0.7000                                                                       n.sub.2  = 1.88300                                                                           ν.sub.2  = 40.78                        r.sub.4  = -9.9727                                                                      d.sub.4  = 0.4195                                                   r.sub.5  = ∞ (stop)                                                               d.sub.5  = 0.4000                                                   r.sub.6  = -2.0492                                                                      d.sub.6  = 8.7675                                                                       n.sub.3  (gradient index lens)                            r.sub.7  = ∞                                                                      d.sub.7  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.8  = ∞                                                                      d.sub.8  = 1.1918                                                   r.sub.9  = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                        N.sub.0     N.sub.1    N.sub.2    N.sub.3                                     ______________________________________                                        d-line 1.60000  -2.5898 × 10.sup.-2                                                                2.4990 × 10.sup.-4                                                               1.5312 × 10.sup.-5                  C-line 1.59640  -2.5862 × 10.sup.-2                                                                2.4990 × 10.sup.-4                                                               1.5312 × 10.sup.-5                  F-line 1.60840  -2.5981 × 10.sup.-2                                                                2.4990 × 10.sup.-4                                                               1.5312 × 10.sup.-5                  ______________________________________                                    

1/V₀ 32 2.0×10⁻², 1/V₁₌ 0.46×10⁻², N₁ ×f² =-0.338

t_(G) /f=2.43, N₂ ×f⁴ =0.042, ν_(p) =40.78

Embodiment 45

f=3.5 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = 6.4658                                                                       d.sub.1  = 0.9000                                                                       n.sub.1  = 1.72916                                                                           ν.sub.1  = 54.68                        r.sub.2  = 1.9642                                                                       d.sub.2  = 1.0000                                                   r.sub.3  = ∞                                                                      d.sub.3  = 1.8004                                                                       n.sub.2  = 1.52542                                                                           ν.sub.2  = 55.78                        r.sub.4  = ∞ (reflecting surface)                                                 d.sub.4  = 2.4999                                                                       n.sub.3  = 1.52542                                                                           ν.sub.3  = 55.78                        r.sub.5  = 3.2080                                                                       d.sub.5  = 1.2553                                                   r.sub.6  = ∞ (stop)                                                               d.sub.6  = 1.5226                                                   r.sub.7  = ∞                                                                      d.sub.7  = 10.1277                                                                      n.sub.4  (gradient index lens)                            r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.5  = 1.48749                                                                           ν.sub.5  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1900                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                               N.sub.0 N.sub.1      N.sub.2                                           ______________________________________                                        d-line   1.67000   -9.1652 × 10.sup.-3                                                                  6.7452 × 10.sup.-5                      C-line   1.66553   -9.2010 × 10.sup.-3                                                                  6.7452 × 10.sup.-5                      F-line   1.68042   -9.0817 × 10.sup.-3                                                                  6.7452 × 10.sup.-5                      ______________________________________                                    

(Focusing)

    ______________________________________                                        Object distance infinite                                                                        Object distance 100 mm                                      ______________________________________                                        d.sub.2 1.0           1.1694                                                  ______________________________________                                    

1/V₀ =2.22×10⁻², 1/V₁ =-1.30×10⁻², N₁ ×f² =-0.112

t_(G) /f=2.89, N₂ ×f⁴ =0.010, ν_(p) /ν_(n=) 1.02

N_(p) /N_(n=) 0.882, ν_(p) =55.78

Embodiment 46

f=3.0 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = 15.6924                                                                      d.sub.1 = 0.7000                                                                        n.sub.1 = 1.77250                                                                            ν.sub.1 = 49.60                         r.sub.2 = 2.6149 (aspherical surface)                                                 d.sub.2 = 2.8443                                                      r.sub.3 = ∞ (reflecting surface)                                                  d.sub.3 = 2.5448                                                    r.sub.4 = -8.8540                                                                       d.sub.4 = 0.8000                                                                        n.sub.2 = 1.84666                                                                            ν.sub.2 = 23.78                         r.sub.5 = 84.7224                                                                       d.sub.5 = 0.8027                                                    r.sub.6 = ∞ (stop)                                                                d.sub.6 = 1.0477                                                    r.sub.7 = -15.7620                                                                    d.sub.7 = 16.8990                                                                     n.sub.3 (gradient index lens)                                 r.sub.8 = -141,4639                                                                     d.sub.8 = 1.0000                                                    r.sub.9 = ∞                                                                       d.sub.9 = 0.7500                                                                        n.sub.4 = 1.48749                                                                            ν.sub.4 = 70.21                         r.sub.10 = ∞                                                                      d.sub.10 = 1.1900                                                   r.sub.11 = ∞ (image)                                                    ______________________________________                                    

Aspherical surface coefficient

A₄ =-6.6412×10⁻³, A₆ =4.1496×10⁻³, A₈ =-2.0583×10⁻⁴

Gradient index lens

    ______________________________________                                               N.sub.0 N.sub.1      N.sub.2                                           ______________________________________                                        d-line   1.70000   -6.4846 × 10.sup.-3                                                                  6.6853 × 10.sup.-5                      C-line   1.69400   -6.5041 × 10.sup.-3                                                                  6.6853 × 10.sup.-5                      F-line   1.71400   -6.4392 × 10.sup.-3                                                                  6.6853 × 10.sup.-5                      ______________________________________                                    

1/V₀ =2.86×10⁻², 1/V₁ =-0.02×10⁻², N₁ ×f² =-0.058

t_(G) /f=5.63, N₂ ×f⁴ =0.005, ν_(p) /ν_(n) =0.45

N_(p) /N_(n) =1.042, ν_(p) =23.78

Embodiment 47

f=5.18 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = -6.0804                                                                      d.sub.1  = 2.5000                                                                       n.sub.1  = 1.52542                                                                           ν.sub.1  = 55.78                        r.sub.2  = ∞ (reflecting surface)                                                 d.sub.2  = 2.5000                                                                       n.sub.2  = 1.52542                                                                           ν.sub.2  = 55.78                        r.sub.3  = -5.9753                                                                      d.sub.3  = 1.2000                                                   r.sub.4  = ∞ (stop)                                                               d.sub.4  = 1.0000                                                   r.sub.5  = ∞                                                                      d.sub.5  = 18.3846                                                                      n.sub.3  (gradient index lens)                            r.sub.6  = ∞                                                                      d.sub.6  = 4.0000                                                   r.sub.7  = ∞ (reflecting surface)                                                 d.sub.7  = 3.5000                                                   r.sub.8  = ∞                                                                      d.sub.8  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.9  = ∞                                                                      d.sub.9  = 1.1489                                                   r.sub.10  = ∞ (image)                                                   ______________________________________                                    

Gradient index lens

    ______________________________________                                               N.sub.0 N.sub.1      N.sub.2                                           ______________________________________                                        d-line   1.60000   -5.0000 × 10.sup.-3                                                                  1.2259 × 10.sup.-6                      C-line   1.59710   -4.9964 × 10.sup.-3                                                                  2.2259 × 10.sup.-6                      F-line   1.60677   -5.0083 × 10.sup.-3                                                                  1.2259 × 10.sup.-6                      ______________________________________                                    

¹ /V₀ =1.61×10⁻² ₁, 1/Vi=0.24×10⁻², N₁ ×f² =-0.134

t_(G) /f=3.55, N₂ ×f⁴ =0.0009

Embodiment 48

f=5.0 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1  = ∞ (stop)                                                               d.sub.1  = 0.5000                                                   r.sub.2  = 33.5152                                                                      d.sub.2  = 3.2126                                                                       n.sub.1  = 1.74100                                                                           ν.sub.1  = 52.65                        r.sub.3  = ∞ (reflecting surface)                                                 d.sub.3  = 3.5126                                                                       n.sub.2  = 1.74100                                                                           ν.sub.2  = 52.65                        r.sub.4  = 9.2424                                                                       d.sub.4  = 0.4000                                                   r.sub.5  = ∞                                                                      d.sub.5  = 5.1185                                                                       n.sub.3  (gradient index lens)                            r.sub.6  = ∞                                                                      d.sub.6  = 0.3287                                                   r.sub.7  = ∞                                                                      d.sub.7  = 0.7500                                                                       n.sub.4  = 1.48749                                                                           ν.sub.4  = 70.21                        r.sub.8  = ∞                                                                      d.sub.8  = 1.1931                                                   r.sub.9  = ∞ (image)                                                    ______________________________________                                    

Gradient index lens

    ______________________________________                                                N.sub.0 N.sub.1     N.sub.2                                           ______________________________________                                        d-line    1.67000   -1.5280 × 10.sup.-2                                                                 2.8237 × 10.sup.-5                      C-line    1.66553   -1.5346 × 10.sup.-2                                                                 2.8237 × 10.sup.-5                      F-line    1.68042   -1.5126 × 10.sup.-2                                                                 2.8237 × 10.sup.-5                      ______________________________________                                    

1/V₀ =2.22×10⁻², 1/V₁ =1.44×10⁻², N₁ ×f² =-0.382

t_(G) /f=1.02, N₂ ×f⁴ =0.018

Embodiment 49

f=3.25 mm, F-number=2.8, image height=1.85 mm, 2ω=62.8°

In case the first lens unit is consisting of the reflecting surfaces

f=6.31 mm, F-number=2.8, image height=1.85 mm,

    ______________________________________                                        r.sub.1 = 9.9669                                                              d.sub.1 = 1.0000                                                                              n.sub.1 = 1.69680                                                                          ν.sub.1 = 55.53                               r.sub.2 = 2.4193 (aspherical surface)                                         d.sub.2 = 1.2000                                                              r.sub.3 = -67.8885                                                            d.sub.3 = 2.8884                                                                              n.sub.2 = 1.52542                                                                          ν.sub.2 = 55.78                               r.sub.4 = ∞ (reflecting surface)                                        d.sub.4 = 3.8741                                                                              n.sub.3 = 1.52542                                                                          ν.sub.3 = 55.78                               r.sub.5 = 5.6750 (aspherical surface)                                         d.sub.5 = 1.9707                                                              r.sub.6 = ∞ (stop)                                                      d.sub.6 = 1.3980                                                              r.sub.7 = ∞                                                             d.sub.7 = 13.1793                                                                             n.sub.4 (gradient index lens)                                 r.sub.8 = ∞                                                             d.sub.8 = 0.7500                                                                              n.sub.5 = 1.48749                                                                          ν.sub.5 = 70.21                               r.sub.9 = ∞                                                             d.sub.9 = 1.1887                                                              r.sub.10 = ∞ (image)                                                    ______________________________________                                    

Aspherical coefficient

(second surface)

A₄ --3.3290×10⁻³, A₆ =-8.0505×10⁻⁴, A₆ =-2.8153×10⁻⁴

(fifth surface)

A₄ =-2.6838×10⁻⁴, A₆ =3.7978×10⁻⁴, A₆ =0

Gradient index lens

    ______________________________________                                               N.sub.0 N.sub.1      N.sub.2                                           ______________________________________                                        d-line   1.67000   -8.1099 × 10.sup.-3                                                                  1.0747 × 10.sup.-5                      C-line   1.66553   -8.1110 × 10.sup.-3                                                                  1.0747 × 10.sup.-5                      F-line   1.68042   -8.1073 × 10.sup.-3                                                                  1.0747 × 10.sup.-5                      ______________________________________                                    

1/V₀ =2.22×10⁻², 1/V₁ =-0.05×10⁻², N₁ ×f² =-0.086

t_(G) /f=4.06, N₂ ×f⁴ =0.012, ν_(p) /ν_(f) --1.00

N_(p) /N_(n) =0.899, ν_(p) =55.78

Embodiment 50

f=6.56˜4.71˜18.91 mm, F-number=3.6˜4.7˜6.2,

image height=3.5 mm, 2ω=60.4°˜36.0°˜20.0°

    ______________________________________                                        r.sub.1  = ∞                                                                      d.sub.1  = 0.8000                                                                          n.sub.1  = 1.81600                                                                        ν.sub.1  = 46.62                        r.sub.2  = 5.6195                                                                       d.sub.2  = 0.4788                                                   r.sub.3  = 6.1296                                                                       d.sub.3  = 0.9983                                                                          n.sub.2  = 1.84666                                                                        ν.sub.2  = 23.78                        r.sub.4  = 11.5281                                                                      d.sub.4  = D.sub.1  (variable)                                      r.sub.5  = ∞ (stop)                                                               d.sub.5  = 0.5000                                                   r.sub.6  = ∞                                                                      d.sub.6  = 5.9864                                                                          n.sub.3  = (gradient                                                          index lens)                                            r.sub.7  = ∞                                                                      d.sub.7  = D.sub.2  (variable)                                      r.sub.8  = 11.6948                                                                      d.sub.8  = 0.8000                                                                          n.sub.4  = 1.74077                                                                        ν.sub.4  = 27.79                        r.sub.9  = 5.7435                                                                       d.sub.9  = D.sub.3  (variable)                                      r.sub.10  = 15.2627                                                                     d.sub.10  = 1.4009                                                                         n.sub.5  = 1.88300                                                                        ν.sub.5  = 40.76                        r.sub.11  = -26.2228                                                                    d.sub.11  = 0.1993                                                  r.sub.12  = ∞                                                                     d.sub.12  = 1.6000                                                                         n.sub.6  = 1.51633                                                                        ν.sub.6  = 64.14                        r.sub.13  = ∞                                                                     d.sub.13  = 1.6000                                                                         n.sub.7  = 1.51633                                                                        ν.sub.7  = 64.14                        r.sub.14  = ∞                                                                     d.sub.14  = 0.4000                                                  r.sub.15  = ∞                                                                     d.sub.15  = 0.7500                                                                         n.sub.8  = 1.51633                                                                        ν.sub.6  = 64.14                        r.sub.16  = ∞                                                                     d.sub.16  = 1.1255                                                  r.sub.17  = ∞ (image)                                                   ______________________________________                                    

    ______________________________________                                        f         6.56        4.71       18.91                                        ______________________________________                                        D.sub.1   11.06493    5.95704    0.50000                                      D.sub.2   5.81906     6.47619    11.96499                                     D.sub.3   1.82166     6.33050    6.36121                                      ______________________________________                                    

Gradient index lens

    ______________________________________                                                    N.sub.0   N.sub.1                                                 ______________________________________                                        d-line        1.65000     -9.2600 × 10.sup.-3                           C-line        1.64512     -9.2557 × 10.sup.-3                           F-line        1.66138     -9.2700 × 10.sup.-3                           ______________________________________                                    

1/V₀ =2.5×10⁻², 1/V₁ =1.5×10⁻³, N₁ ×f² =-0.398

t_(G) /f=0.913, N₂ ×f⁴ =0,

wherein the reference symbols r₁, r₂, . . . represent radii of curvatureon surfaces of respective lens components, the reference symbols d₁, d₂,. . . designate thicknesses of the respective lens components andairspaces reserved therebetween, the reference symbols n₁, n₂, . . .denote refractive indices of the respective lens components for thed-line, and the reference symbols ν₁, ν₂ . . . , represent Abbe'snumbers of the respective lens components.

The first through seventeenth embodiments of the present invention havethe first composition, or are composed as described in detail below:

The first embodiment has a composition illustrated in FIG. 1, or iscomposed, in order from the object side, of a positive lens component, anegative lens component, a positive lens component and a positive lenscomponent. Speaking more concretely, the first embodiment is composed,in order from the object side, of a first lens unit which consists of apositive meniscus lens component having a concave surface on the imageside and a negative meniscus lens component having a concave surface onthe image side, an aperture stop S, and a second lens unit whichconsists of a positive meniscus lens component having a concave surfaceon the object side and a positive plano-convex lens component. The firstembodiment is a retrofocus type lens system in which the plano-convexlens component disposed on the image side is configured as a radial typegradient index lens component. In FIG. 1, the reference symbol LArepresents a axial ray, the reference symbol LB designates an offaxialray, the reference symbol C denotes a planer protective glass plate foran image pickup device such as a CCD or a CMD, the reference symbol Srepresents an aperture stop and the reference symbol SF designates aflare stop.

Though the first embodiment is composed of a small number of lenscomponents, it favorably corrects lateral chromatic aberration bydisposing the negative lens component and the positive lens component inthe first lens unit, and using the radial type gradient index lenscomponent in the second lens unit. Further, the first embodiment hasrealized nearly telecentric incidence of the offaxial ray onto an imagesurface relatively easily by configuring an image side surface as aconvex surface.

By selecting the positive lens component as the object side lenscomponent on which the offaxial ray is highest, or by composing thefirst lens unit of the positive lens component and the negative lenscomponent, the first embodiment moderates asymmetry of the lens system,thereby favorably correcting lateral chromatic aberration and distortionwith the small number of lens components. Further, amounts ofaberrations are reduced by composing the second lens unit of the twopositive lens components for sharing a positive refractive power betweenthese two lens components. Offaxial aberrations are corrected favorablyby adopting the radial type gradient index lens component as the imageside lens component of the second lens unit on which the offaxial ray ishigh. The flare stop SF for cutting off offaxial flare components isdisposed on the object side of the aperture stop S.

The first embodiment is an example wherein the manufacturing cost of thelens system is reduced by configuring one surface of the radial typegradient index lens component as a planar surface which can easily bepolished. In the retrofocus type lens system preferred as the firstembodiment, longitudinal chromatic aberration and spherical aberrationproduced in the lens system as a whole are corrected by the concave lenscomponent disposed in the first lens unit.

Though the lens system preferred as the first embodiment is composedonly of the four lens components, aberrations are corrected favorably inthis lens system.

The second embodiment has a composition illustrated in FIG. 2, or iscomposed, in order from the object side, of a positive lens component, anegative lens component, a positive lens component and a positive lenscomponent. Speaking more concretely, the second embodiment is composed,in order from the object side, of a first lens unit which consists of apositive meniscus lens component having a concave surface on the imageside and a negative meniscus lens component having a concave surface onthe image side, and a second lens unit which consists of a positivemeniscus lens component having a concave surface on the object side anda positive biconvex lens component: the lens component disposed on theobject side in the second lens unit being a radial type gradient indexlens component. An aperture stop is disposed at a location 0.3648 mm asmeasured from the object side surface of the radial type gradient indexlens component. Further, a flare stop SF for cutting off offaxial flarecomponents is disposed at a location 0.96 mm as measured from the objectside surface of the radial type gradient index lens component.

In the second embodiment, the radial type gradient index lens componenthas a shortened diameter and is disposed on the object side in thesecond lens unit for reducing the manufacturing cost of the lens system.By reducing the diameter of the gradient index lens component, it ispossible to shorten the time required for imparting a refractive indexdistribution, thereby lowering a manufacturing cost of the lens system.The aperture stop is disposed in the radial type gradient index lenscomponent for lowering offaxial rays passing through this lenscomponent, thereby making it possible to reduce a diameter of this lenscomponent. Furthermore, offaxial aberrations in particular are correctedfavorably by configuring the lens component disposed on the image sideas a biconvex lens component for sharing a positive refractive powerbetween both surfaces of this lens component.

A retrofocus type lens system using a first lens unit which is composed,in order from the object side, of a positive lens component and anegative lens component like the second embodiment, is suitedparticularly to have a widened field angle.

The lens system preferred as the second embodiment is focused onto anobject located at an extremely short distance by moving the first lenscomponent toward the object side. However, this lens system can befocused onto the object located at the extremely short distance bymoving the lens system as a whole toward the object side.

Though the lens system preferred as the second embodiment is composedonly of the four lens components, aberrations are corrected favorably inthis lens system.

The third t is composed, in order from the object side, of a positivelens component, a negative lens component and a positive lens componentas shown in FIG. 3. Speaking more concretely, the third embodiment iscomposed, in order from the object side, of a first lens unit whichconsists of a positive biconvex lens component and a negative meniscuslens component having a concave surface on the image side, a stop S, anda second lens unit which consists of a positive plano-convex lenscomponent: the lens component disposed in the second lens unit being aradial type gradient index lens component. Further, plain parallelplates F₁ and F₂ are two kinds of filters for cutting off componentshaving specific wave lengths. Out of these plain parallel plates, F₁ isan infrared cut filter and F₂ is a low pass filter.

The first lens unit of the third embodiment is composed, in order fromthe object side, of the positive lens component and the negative lenscomponent for making it possible to favorably correct lateral chromaticaberration and distortion. The second lens unit which is composed of thesingle radial type gradient index lens component is configured so as tosatisfy the conditions (1) and (2) for favorable correction of lateralchromatic aberration and a Petzval's sum in particular.

For nearly telecentric incidence of offaxial rays onto an image surface,the positive lens component disposed in the second lens unit isconfigured so as to have a convex surface on the image side. Though thissurface tends to produce coma in a large amount, coma is correctedfavorably by configuring this lens component as the radial type gradientindex lens component and so as to satisfy the condition (2).

Further, the radial type gradient index lens component has a planarsurface on one side for reducing the cost required for polishing.

Aberrations are corrected favorably in the lens system preferred as thethird embodiment which is composed only of the three lens components.

The fourth embodiment is composed, in order from the object side, of apositive lens component, a negative lens component and a positive lenscomponent as shown in FIG. 4, or has a composition which issubstantially the same as that of the third embodiment. Speaking moreconcretely, the fourth embodiment is composed, in order from the objectside, of a first lens unit which consists of a positive meniscus lenscomponent having a concave surface on the image side and a negativemeniscus lens component having a concave surface on the image side, astop S, and a second lens unit which consists of a positive plano-convexlens component: the lens component disposed in the second lens unitbeing a radial type gradient index lens component. Furthermore, a planeparallel plate F₂ functions as a cut filter which cuts off componentshaving specific wavelengths.

The fourth embodiment is an example in which at least one lens componenthas a filter function for eliminating components having specificwavelengths, whereby filters are used in a number one smaller than thatof the filters used in the third embodiment. In this embodiment, theradial type gradient index lens component, for example, has a functionto cut off components having wavelengths within the infrared region foromitting the infrared cut filter in the third embodiment, therebyreducing a manufacturing cost of the lens system and shortening a totallength thereof.

The fifth embodiment is composed, in order from the object side, of apositive lens component, a negative lens component and a positive lenscomponent as shown in FIG. 5 or has a composition which is substantiallythe same as that of the third embodiment. Speaking more concretely, thefifth embodiment is composed, in order from the object side, of a firstlens unit which consists of a positive meniscus lens component having aconcave surface on the image side and a negative meniscus lens componenthaving a concave surface on the image side, an aperture stop S, and asecond lens unit which consists of a plano-convex lens component: thelens component disposed in the second lens unit being a radial typegradient index lens component.

The fifth embodiment is an example in which at least one lens componenthas filter functions for eliminating components having specificwavelength, whereby filters are used in a number two smaller than thatof the filters used in the third embodiment for configuring the lenssystem to be more compact and reducing its manufacturing cost.

The fifth embodiment is focused onto an object located at an extremelyshort distance by moving toward the object side the first lens unitwhich is disposed on the object side of the aperture stop.

The sixth embodiment is composed, in order from the object side, of apositive lens component, a negative lens component and a positive lenscomponent as shown in FIG. 6A. Speaking more concretely, the sixthembodiment is composed, in order from the object side, of a first lensunit which consists of a positive meniscus lens component having aconcave surface on the image side and a negative meniscus lens componenthaving a concave surface on the image side, and a second lens unit whichconsists of a plano-convex lens component: the lens component disposedin the second lens unit being a radial type gradient index lenscomponent. An aperture stop is disposed at a location 2.1619 mm asmeasured from an object side surface of the radial type gradient indexlens component.

In the sixth embodiment, the aperture stop is disposed in the radialtype gradient index lens component, thereby reducing a diameter of theradial type gradient index lens component and lowering a manufacturingcost of the lens system. Further, the disposition of the aperture stopin the radial type gradient index lens component produces an effect tomoderate asymmetry of the lens system, thereby facilitating correctionof aberrations. When an aperture stop is to be disposed in a lenscomponent as in the sixth embodiment, an aperture stop can actually beprovided by forming a cut from an outer circumference 30 of the lenscomponent toward the optical axis as shown on an enlarged scale in FIG.6B. It is desirable to coat this cut with a black paint or finish thecut surfaces in satin-like surface so that the cut can shield rays.Further, an aperture stop can be formed in a lens component by anothermethod, or cutting a lens component into two pieces, disposing anaperture stop on a cut surface and cementing the two pieces to eachother or bringing them into close contact with each other. This methodis applicable also to formation of a flare stop.

When a radial type gradient index lens component is to be manufacturedby the sol-gel method, for example, an aperture stop can be disposed atthe manufacturing stage.

The seventh embodiment is composed, in order from the object side, of apositive lens component, a negative lens component and a positive lenscomponent as shown in FIG. 7A. Speaking more concretely, the seventhembodiment is composed, in order from the object side, of a first lensunit which consists of a positive convexo-planar lens component and apositive meniscus lens component having a concave surface on the objectside: the lens component disposed in the second lens unit being a radialtype gradient index lens component. The seventh embodiment is an exampleof an attempt to configure the lens system to be compact by configuringthe radial type gradient index lens component so as to have a largedifference in refractive index between a portion on the optical axis anda marginal portion, and thinning this lens component. An aperture stopis disposed on an object side surface of the radial type gradient indexlens component. For disposing an aperture stop on a lens surface as inthe seventh embodiment, a thin sheet D is cemented to the surface orbringing the thin sheet D into close contact with the surface as shownin FIG. 7B. Further, an aperture stop can be formed by finishing a lenssurface as a satin surface or coating it with a black paint.

The eighth embodiment is composed, in order from the object side, of apositive lens component, a negative lens component and a positive lenscomponent as shown in FIG. 8. Speaking more concretely, the eighthembodiment is composed, in order from the object side, of a first lensunit which consists of a positive meniscus lens component having aconcave surface on the object side and a negative meniscus lenscomponent having a concave surface on the image side, an aperture stopS, and a second lens unit which consists of a positive piano-convex lenscomponent: the lens disposed in the second lens unit being a radial typegradient index lens component.

Further, the eighth embodiment uses an aspherical surface as a firstsurface, thereby favorably correcting distortion in particular. A shapeof this aspherical surface and those of aspherical surfaces used in thepresent invention are expressed by the following formula: ##EQU1##wherein a direction along the x axis is taken as the z axis, a directionperpendicular to the optical axis is taken as the y axis, the referencesymbol r represents a radius of curvature on the optical axis and thereference symbol Ai designates an aspherical surface coefficient.

The ninth embodiment has a composition illustrated in FIG. 9, or iscomposed of a positive lens component, a negative lens component and apositive lens component in order from the object side. Speaking moreconcretely, the ninth embodiment is composed, in order from the objectside, of a first lens unit which consists of a positive meniscus lenscomponent having a concave surface on the image side and a negativemeniscus lens component having a concave surface on the image side, anda second lens unit which consists of a positive meniscus lens componenthaving a concave surface on the object side: the lens component disposedin the second lens unit being a radial type gradient index lenscomponent. Further, an aperture stop is disposed at a location 1.2232 mmas measured from the object side surface of the radial type gradientindex lens component.

The ninth embodiment is an example wherein an image side surface of theradial type gradient index lens component is configured as an asphericalsurface which has such a shape as to weaken a positive refractive poweras portions of the aspherical surface are farther from the optical axisin a radial direction, whereby coma and distortion produced by thissurface are corrected favorably.

In the ninth embodiment, the effect of the aspherical surface isobtained by cementing an aspherical transparent member T to the radialtype gradient index lens component or bringing the member T into closecontact with the lens component. This transparent member T is made of aresin, a plastic material, a glass material, a crystal or a liquid. Asimilar effect can be obtained by forming an aspherical surface directlyon the radial type gradient index lens component.

When coma or distortion in particular is to be corrected by using anaspherical surface on a lens component disposed on the object side ofthe aperture stop, it is desirable to configure the aspherical surfaceso as to have a shape which weakens a negative refractive power asportions of the aspherical surface are farther from the optical axistoward a margin. When an aspherical surface is to be used on a lenscomponent disposed on the image side of the aperture stop, it isdesirable to configure an aspherical surface so as to have a shape whichweakens a positive refractive power as portions of the asphericalsurface are farther from the optical axis toward the margin.

The lens system preferred as the ninth embodiment is focused onto anobject located at an extremely short distance by moving the lens systemas a whole toward the object side.

The tenth embodiment is composed, in order from the object side, of anegative lens component, a positive lens component, a positive lenscomponent and a positive lens component as shown in FIG. 10. Speakingmore concretely, the tenth embodiment is composed, in order from theobject side, of a first lens unit which consists of a negative biconcavelens component and a positive biconvex lens component, an aperture stopS, and a second lens unit which consists of a positive meniscus lenscomponent having a concave surface on the object side and a positiveconvexo-planar lens component: the lens component disposed on the objectside in the second lens unit being a radial type gradient index lenscomponent.

Though the tenth embodiment is composed of a small number of lenscomponents, it is configured so as to favorably correct lateralchromatic aberration in particular by using the negative lens componentand the positive lens component in the first lens unit, and the radialtype gradient index lens component in the second lens unit. Further, acomposition advantageous for widening a field angle of the lens systemwas obtained by comprising the first lens unit of the negative lenscomponent and the positive lens component, in that order from the objectside. Though the composition using the negative lens component as thefirst lens component on which offaxial rays are high makes it relativelyhard to correct lateral chromatic aberration, lateral chromaticaberration is corrected favorably in this embodiment by configuring theradial type gradient index lens component so as to satisfy the condition(1).

Further, nearly telecentric incidence of the offaxial rays onto an imagesurface is made possible relatively easily by selecting the convexsurface as the image side surface of the radial type gradient index lenscomponent. In addition, coma and distortion are corrected favorably byconfiguring the radial type gradient index lens component so as tosatisfy the condition (2). In other words, the radial type gradientindex lens component has refractive indices which are lowered as theportions of the convex surface are farther from the optical axis towardthe margin when it satisfies the condition (2), thereby making itpossible to favorably correct coma and distortion produced by thissurface. The tenth embodiment has a planar image side surface and allowsa protective glass plate to be attached to this surface. Moreover, it ispossible to omit the protective glass plate C by configuring the imageside lens component as a protective glass plate.

The tenth embodiment is focused onto an object located at an extremelyshort distance by moving the object side negative lens component towardthe object side.

Though the lens system preferred as the tenth embodiment is composedonly of the lens component in a small number (four), aberrations arecorrected favorably in this embodiment.

The eleventh embodiment has a composition illustrated in FIG. 11, or iscomposed of a negative lens component, a positive lens component and apositive lens component, in that order from the object side. Speakingmore concretely, the eleventh embodiment is composed, in order from theobject side, of a first lens unit which consists of a negative biconcavelens component and a positive biconvex lens component, an aperture stop,and a second lens unit which consists of a positive meniscus lenscomponent having a concave surface on the object side: the lenscomponent disposed in the second lens unit being a radial type gradientindex lens component.

The eleventh embodiment in which the first lens unit is composed, inorder from the object side, of the negative lens component and thepositive lens component has a composition which is advantageous forwidening a field angle of the lens system. Though the eleventhembodiment is composed only of the three lens components, aberrationsare corrected favorably in this embodiment by configuring the radialtype gradient index lens component so as to satisfy the conditions (1)and (2).

For configuring compactly the so-called retrofocus type lens systemwhich uses at least one negative lens component in a first lens unit anda second lens unit having a positive refractive power, it is desirableto configure a radial type gradient index lens component so as tosatisfy the following condition (18):

    1<t.sub.G /f<6                                             (18)

If the lower limit of 1 of the condition (18) is not satisfied, theradial type gradient index lens component will have a weak refractivepower of medium, thereby undesirable undercorrecting lateral chromaticaberration. If the upper limit of 6 of the condition (18) is exceeded,in contrast, the radial type gradient index lens component will bethick, thereby undesirably prolonging a total length of the lens system.

Aberrations are corrected favorably in the lens system preferred as theeleventh embodiment though it is composed only of the three lenscomponents.

The twelfth t is composed, in order from the object side, of a negativelens component, a negative lens component and a positive lens componentas shown in FIG. 12. Speaking more concretely, the twelfth embodiment iscomposed, in order from the object side, of a first lens unit whichconsists of a negative meniscus lens component having a concave surfaceon the image side and a negative meniscus lens component having aconcave surface on the image side, an aperture stop S, and a second lensunit which consists of a positive biconvex lens component: the lenscomponent disposed in the second lens unit being a radial type gradientindex lens component.

The twelfth embodiment favorably corrects offaxial aberrations though ithas a field angle which is widened by composing the first lens unit ofthe two negative lens components for sharing a negative refractive powerbetween these lens components.

The thirteenth embodiment is composed, in order from the object side, ofa negative lens component, a positive lens component and a positive lenscomponent as shown in FIG. 13. Speaking more concretely, the thirteenthembodiment is composed, in order from the object side, of a first lensunit which consists of a negative meniscus lens component having aconcave surface on the image side, an aperture stop, and a second lensunit which consists of a positive meniscus lens component having aconcave surface on the object side and a positive meniscus lenscomponent having a concave surface on the object side: the lenscomponent disposed on the object side in the second lens unit being aradial type gradient index lens component. Further, the image sidesurface of the first lens component is configured as an asphericalsurface which weakens a negative refractive power as portions of theaspherical surface are farther from the optical axis in the radialdirection and serves for correcting distortion.

Though the thirteenth embodiment uses the first lens unit which iscomposed only of the negative lens component, it corrects aberrationsfavorably using the radial type gradient index lens component.

The fourteenth embodiment is composed, in order from the object side, ofa negative lens component, a positive lens comment and a negative lenscomponent as shown in FIG. 14. Speaking more concretely, the fourteenthembodiment is composed, in order from the object side, of a first lensunit which consists of a negative meniscus lens component having aconcave surface on the image side, an aperture stop, and a second lensunit which consists of a positive meniscus lens component having aconcave surface on the object side and a negative meniscus lenscomponent having a concave surface on the image side: the lens componentdisposed on the object side in the second lens unit being a radial typegradient index lens component.

The fourteenth embodiment adopts a negative lens component as the imageside lens component for moderating asymmetry of the lens system as awhole, thereby favorably correcting offaxial aberrations.

The fifteenth embodiment has a composition illustrated in FIG. 15, or iscomposed of a negative lens component, a negative lens component and apositive lens component in that order from the object side. Speakingmore concretely, the fifteenth embodiment is composed, in order from theobject side, of a first lens unit which consists of a negative meniscuslens component having a concave surface on the image side, an aperturestop, and a second lens unit which consists of a negative meniscus lenscomponent having a concave surface on the image side and a positivemeniscus lens component having a concave surface on the object side: thelens component disposed on the image side in the second lens unit beinga radial type gradient index lens component. The fifteenth embodimentmoderates asymmetry of the lens system as a whole by using the negativelens component in the second lens unit, thereby favorably correctingoffaxial aberrations. Further, this embodiment favorably correctslateral chromatic aberration and distortion by disposing the radial typegradient index lens component at the image side location.

The sixteenth embodiment has a composition illustrated in FIG. 16, or iscomposed of a negative lens component, a positive lens component and apositive lens component in that order from the object side. Speakingmore concretely, the sixteenth embodiment is composed, in order from theobject side, of a first lens unit which consists of a negative meniscuslens component having a concave surface on the image side, an aperturestop S, and a second lens unit which consists of a positive meniscuslens component having a concave surface on the object side and apositive meniscus lens component having a concave surface on the objectside: the lens component disposed on the image side in the second lensunit being a radial type gradient index lens component. In the sixteenthembodiment, aberrations are corrected favorably by composing the secondlens unit of the two positive lens components for sharing a refractivepower between these lens components.

The seventeenth embodiment is composed, in order from the object side,of a negative lens component and a positive lens component as shown inFIG. 17. Speaking more concretely, the seventeenth embodiment iscomposed, in order from the object side, of a first lens unit whichconsists of a negative meniscus lens component having a concave surfaceon the image side, an aperture stop S and a second lens unit whichconsists of a positive piano-convex lens component: the lens componentdisposed in the second lens unit being a radial type gradient index lenscomponent.

Though the seventeenth embodiment is composed only of the two lenscomponents, it is a lens system which has favorably correctedaberrations and can be manufactured at a low cost.

Each of the eighteenth through thirty-seventh embodiment is a lenssystem according to the present invention which has the secondcomposition and uses an aperture stop disposed on the object side of thelens system. These embodiments will be described in detail below:

The eighteenth embodiment has a composition illustrated in FIG. 18, oris composed of a positive lens component, a negative lens element, apositive lens element and a positive lens component. Speaking moreconcretely, the eighteenth --embodiment is composed, in order from theobject side, of an aperture stop, a positive lens component having aconcave surface on the object side, a cemented lens component whichconsists of a negative biconcave lens element and a positive biconvexlens element, and a positive meniscus lens component having a convavesurface on the object side: the lens component disposed on the imageside being a radial type gradient index lens component.

The eighteenth embodiment is an example wherein nearly telecentricincidence of offaxial rays on an image surface is facilitated bydisposing the aperture stop on the object side. Further, lateralchromatic aberration is corrected favorably by using the radial typegradient index lens component which satisfies the condition (1).Furthermore, the radial type gradient index lens component is configuredso as to satisfy the condition (2) for favorably correcting a Petzval'ssum. The cemented lens component (the negative lens element and thepositive lens element) is configured so as to satisfy the condition(14), thereby favorably correcting the Petzval's sum. Moreover, thePetzval's sum in particular is favorably corrected by configuring theradial type gradient index lens component so as to have the meniscusshape and a weak power of surface. In addition, the radial type gradientindex lens component is configured so as to have the meniscus shapewhich has the concave surface on the side of the stop, thereby making itpossible to favorably correct offaxial aberrations.

The nineteenth embodiment has a composition illustrated in FIG. 19, oris composed of a positive lens component, a positive lens component, anegative lens element and a positive lens element. Speaking moredetailedly, the nineteenth embodiment is composed, in order from theobject side, of an aperture stop, a positive biconvex lens component, apositive meniscus lens component having a concave surface on the objectside, and a cemented lens component which consists of a negativemeniscus lens element having a concave surface on the image side and apositive meniscus lens element having a concave surface on the imageside: the second lens component being a radial type gradient index lenscomponent. An object side surface of the first lens component isconfigured as an aspherical surface. In the nineteenth embodiment,lateral chromatic aberration in particular is corrected favorably bydisposing the cemented lens component on the image side where offaxialrays are highest.

The twentieth embodiment is composed, in order from the object side, ofa positive lens component, a positive lens component, a positive lenselement and a negative lens element as shown in FIG. 20. In detail, thetwentieth embodiment is composed, in order from the object side, of anaperture stop, a positive biconvex lens component, a positive meniscuslens component having a concave surface on the object side, and acemented lens component which consists of a positive biconvex lenselement and a negative biconcave lens element: the second lens componentbeing a radial type gradient index lens component.

The twentieth embodiment is an example wherein lateral chromaticaberration is corrected more favorably by disposing the negative lenselement on the image side.

The twentieth embodiment is focused onto an object located at anextremely short distance by moving the lens system as a whole toward theobject side.

The twenty-first embodiment is composed, in order from the object side,of a positive lens component, a positive lens component and a positivelens component as shown in FIG. 21. Speaking more concretely, thetwenty-first embodiment is composed, in order from the object side, ofan aperture stop, a positive biconvex lens component, a positivemeniscus lens component having a concave surface on the object side anda positive meniscus lens component having a concave surface on the imageside: the positive lens component disposed on the image side being aradial type gradient index lens component.

The twenty-second embodiment has a composition illustrated in FIG. 22,or is composed of a positive lens component, a negative lens componentand a positive lens component. Speaking more detailedly, thetwenty-second embodiment is composed, in order from the object side, ofan aperture stop, a positive meniscus lens component having a concavesurface on the object side, a negative meniscus lens component having aconcave surface on the object side and a positive meniscus lenscomponent having a concave surface on the object side: the lenscomponent disposed on the image side being a radial type gradient indexlens component.

The twenty-third embodiment is composed, in order from the object side,of a positive lens element, a negative lens element and a positive lenselement as shown in FIG. 23. In detail, the twenty-third embodiment iscomposed, in order from the object side, of an aperture stop, a cementedlens component which consists of a positive biconvex lens element and anegative meniscus lens element having a concave surface on the objectside, and positive meniscus lens component having a concave surface onthe object side: the lens component disposed on the image side being aradial type gradient index lens component. The cemented lens componentserves for correcting mainly a Petzval's sum.

The twenty-fourth embodiment is composed, in order from the object side,of a positive lens component, a positive lens component and a positivelens component as shown in FIG. 24. Speaking more concretely, thetwenty-fourth embodiment is composed in order from the object side, ofan aperture stop, a positive biconvex lens component, a positivemeniscus lens component having a concave surface on the object side anda positive meniscus lens component having a concave surface on the imageside: the second lens component being a radial type gradient index lenscomponent.

The twenty-fifth embodiment is composed, in order from the object side,of a positive lens component, a positive lens component and a negativelens component as shown in FIG. 25. Speaking more detailedly, thetwenty-fifth embodiment is composed, in order from the object side, ofan aperture stop, a positive plano-convex lens component, a positivemeniscus lens component having a concave surface on the object side anda negative meniscus lens component having a concave surface on the imageside: the second lens component being a radial type gradient index lenscomponent.

The twenty-sixth embodiment is composed, in order from the object side,of a negative lens element, a positive lens element and a positive lenscomponent as shown in FIG. 26. In detail, the twenty-sixth embodiment iscomposed, in order from the object side, of an aperture stop, a cementedlens component which consists of a negative biconcave lens element and apositive biconvex lens element, and a positive meniscus lens componenthaving a concave surface on the object side: the lens component disposedon the image side being a radial type gradient index lens component.Though the twenty-sixth embodiment is composed only of the three lenselements, it is an example wherein aberrations are corrected favorablyby configuring the lens system so as to satisfy the conditions (1) and(2). Further, a Petzval's sum is favorably corrected mainly by thecemented lens component.

The twenty-seventh embodiment is composed, in order from the objectside, of a negative lens component, a negative lens component and apositive lens component as illustrated in FIG. 27. Speaking moreconcretely, the twenty-seventh embodiment is composed, in order from theobject side, of an aperture stop, a negative meniscus lens componenthaving a concave surface on the image side, a negative meniscus lenscomponent having a concave surface on the object side and a positivebiconvex lens component: the lens component disposed on the image sidebeing a radial type gradient index lens component.

The twenty-eighth embodiment is composed, in order from the object side,of a negative lens component, a positive lens component and a positivelens component as illustrated in FIG. 28. In detail, the twenty-eighthembodiment is composed, in order from the object side, of an aperturestop, a negative meniscus lens component having a concave surface on theimage side, a positive lens component having a concave surface on theobject side and a positive convexo-planar lens component: the secondlens component being a radial type gradient index lens component.

The twenty-ninth embodiment is composed, in order from the object side,of a negative lens component, a positive lens component, and a negativelens component as shown in FIG. 29. Speaking more detailedly, thetwenty-ninth t is composed, in order from the object side, of anaperture stop, a negative meniscus lens component having a concavesurface on the object side, a positive biconvex lens component and anegative meniscus lens component having a concave surface on the imageside: the second lens component being a radial type gradient index lenscomponent.

The thirtieth embodiment is composed, in order from the object side, ofa positive lens component, a positive lens component, and a positivelens component as shown in FIG. 30. Speaking more concretely, thethirtieth embodiment is composed, in order from the object side, of anaperture stop, a positive meniscus lens component having a concavesurface on the object side, a positive meniscus lens component having aconcave surface on the image side and a positive meniscus lens componenthaving a concave surface on the image side: the lens component disposedon the object side being a radial type gradient index lens component.

The thirty-first embodiment has a composition shown in FIG. 31, or iscomposed of a positive lens component, a positive lens element and anegative lens element in order from the object side. Speaking moredetailedly, the thirty-first t is composed, in order from the objectside, of an aperture stop, a positive meniscus lens component having aconcave surface on the object side, and a cemented lens component whichconsists of a positive meniscus lens element having a concave surface onthe object side and a negative biconcave lens element: the lenscomponent disposed on the object side being a radial gradient index lenscomponent. The cemented lens component serves mainly for correctingPetzval's sum favorably.

The thirty-second embodiment has a composition illustrated in FIG. 32,or is composed of a positive lens component, a negative lens element anda positive lens element in order from the object side. In detail, thethirty-second t is composed, in order from the object side, of anaperture stop, a positive meniscus lens component having a concavesurface on the object side, and a cemented lens component which consistsof a negative biconcave lens element and a positive meniscus lenselement having a concave face on the image side: the lens componentdisposed on the object side being a radial type gradient index lenscomponent. The image side surface of the lens element disposed on theimage side is configured as an aspherical surface. The cemented lenscomponent serves mainly for favorably correcting a Petzval's sum.

The thirty-third embodiment has a composition shown in FIG. 33, or iscomposed of a positive lens component, a negative lens element and anegative lens element in order from the object side. Speaking moreconcretely, the thirty-third embodiment is composed, in order from theobject side, of an aperture stop, a positive meniscus lens componenthaving a concave surface on the object side, a negative meniscus lenscomponent having a concave surface on the image side, and a cementedlens component which consists of a negative meniscus lens element havinga concave surface on the image side and a negative meniscus lens elementhaving a concave surface on the image side: the lens component disposedon the object side being a radial type gradient index lens component.

The thirty-fourth embodiment has a composition illustrated in FIG. 34,or is composed of a negative lens component and a positive lenscomponent in that order from the object side. Speaking more detailedly,the thirty-fourth embodiment is composed, in order from the object side,of an aperture stop, a negative meniscus lens component having concavesurface on the image side and a positive meniscus lens component havinga concave surface on the object side: the lens component disposed on theimage side being a radial type gradient index lens component. Though thethirty-fourth embodiment is composed only of the two lens components, itfavorably corrects aberrations by using the radial type gradient indexlens component which satisfies the conditions (1) and (2).

The thirty-fifth embodiment has a composition illustrated in FIG. 35, oris composed of a positive lens component and a positive lens componentin order from the object side. In more detail, the thirty-fifthembodiment is composed, in order from the object side, of an aperturestop, a positive meniscus lens component having a concave surface on theobject side and a positive biconcave lens component: the lens componentdisposed on the image side being a radial type gradient index lenscomponent. In the thirty-fifth embodiment, a Petzval's sum is favorablycorrected mainly by the radial type gradient index lens component whichhas a shape of a negative lens component.

The thirty-sixth embodiment has a composition shown in FIG. 36, or iscomposed of a positive lens component and a negative lens component inthat order from the object side. Speaking more concretely, thethirty-sixth embodiment is composed, in order from the object side, ofan aperture stop, a positive meniscus lens component having a concavesurface on the object side and a negative biconcave lens component: thelens component disposed on the object side being a radial type gradientindex lens component.

The thirty-seventh embodiment has a composition shown in FIG. 37, or iscomposed of a positive lens component and a positive lens component inorder from the object side. Speaking more detailedly, the thirty-seventhembodiment is composed, in order from the object side, of an aperturestop, a positive meniscus lens component having a concave surface on theobject side and a positive meniscus lens component having a concavesurface on the object side: the lens component disposed on the objectside being a radial type gradient index lens component.

Each of the thirty-eighth through forty-fourth embodiments has the thirdcomposition according to the present invention and is configured asdescribed below:

The thirty-eighth t has a composition illustrated in FIG. 38, or iscomposed of a positive lens component, a positive lens component, apositive lens element and a negative lens element in order from theobject side. Speaking more concretely, the thirty-eighth embodiment iscomposed, in order from the object side, of a first lens unit whichconsists of a positive plano-convex lens component, an aperture stop,and a second lens unit which consists of a positive meniscus lenscomponent having a concave surface on the object side, and a cementedlens component consisting of a positive biconvex lens element and anegative biconcave lens element: the second lens component being aradial type gradient index lens component. The thirty-eighth embodimentfavorably corrects lateral chromatic aberration and distortion inparticular owing to the fact that the first lens unit is composed of thesingle lens component. Lateral chromatic aberration is favorablycorrected by configuring the positive lens component disposed in thefirst lens unit so as to satisfy the condition (16). The cemented lenscomponent serves mainly for correcting lateral chromatic aberrationfavorably in the lens system as a whole.

The thirty-eighth embodiment is focused on an object located at anextremely short distance by moving the first lens component toward theobject side.

The thirty-ninth embodiment has a composition shown in FIG. 39, or iscomposed of a positive lens component, a positive lens component and apositive lens component. In more detail, the thirty-ninth embodiment iscomposed, in order from the object side, of a first lens unit whichconsists of a positive biconvex lens component, an aperture stop, and asecond lens unit which consists of a positive meniscus lens componenthaving a concave surface on the object side and a positive lenscomponent having a concave surface on the image side: the second lenscomponent being a radial type gradient index lens component. Anaspherical surface is used as an object side surface of the third lenscomponent.

When the first lens unit of the lens system according to the presentinvention is to be composed of a single positive lens component as inthe thirty-ninth embodiment or when an aperture stop is to be disposedon the object side, it is desirable for configuring the lens system tobe more compact that a radial type gradient index lens componentsatisfies the following condition (19):

    0.5<t.sub.G /f<4                                           (19)

If the lower limit of 0.5 of the condition (19) is not satisfied, theradial type gradient index lens component will have a weak refractivepower of medium, thereby undesirably undercorrecting lateral chromaticaberration. If the upper limit of 4 of the condition (19) is exceeded,the radial type gradient index lens component will be thick, therebyundesirably increasing the total length of the lens system.

The fortieth embodiment has a composition illustrated in FIG. 40, or iscomposed of a positive lens component, a positive lens component and anegative lens component in that order from the object side. Speakingmore detailedly, the fortieth embodiment is composed, in order from theobject side, of a first lens unit which consists of a positive meniscuslens component having a concave surface on the object side, an aperturestop, and a second lens unit which consists of a positive meniscus lenscomponent having a concave surface on the object side and a negativemeniscus lens component having a concave surface on the image side: thesecond lens component being a radial type gradient index lens component.

The forty-first embodiment has a composition shown in FIG. 41, or iscomposed of a positive lens component, a negative lens component and apositive lens component in that order from the object side. Speakingmore concretely, the forty-first embodiment is composed, in order fromthe object side, of a first lens unit which consists of a positivemeniscus lens component having a concave surface on the object side, anaperture stop, and a second lens unit which consists of a negativebiconcave lens component and a positive biconvex lens component: thelens component disposed on the image side being a radial type gradientindex lens component.

The forth-first embodiment is an example wherein a radial type gradientindex lens component has a biconvex shape so as to reduce a differencein refractive index between an optical axis and a margin, therebyfacilitating manufacturing of a material for the radial type gradientindex lens component.

The forty-second embodiment has a composition illustrated in FIG. 42, oris composed of a positive lens component, a positive lens component anda positive lens component, in that order from the object side. In moredetail, the forty-second embodiment is composed, in order from theobject side, of a first lens unit which consists of a positive biconvexlens component, an aperture stop S, and a second lens unit whichconsists of a positive meniscus lens component having a concave surfaceon the object side and a positive lens component having a concavesurface on the image side: the lens component disposed on the image sidebeing a radial type gradient index lens component.

The forty-third embodiment has a composition shown in FIG. 43, or iscomposed of a positive lens component and a positive lens component inorder from the object side. Speaking more detailedly, the forty-thirdembodiment is composed, in order from the object side, of a first lensunit which consists of a positive meniscus lens component having aconcave surface on the object side, an aperture stop S and a second lensunit which consists of a positive meniscus lens component having aconcave surface on the object side: the lens component disposed on theimage side being a radial type gradient index lens component. Though theforty-third embodiment is composed only of the two lens components, itcorrects aberrations favorably by using a radial type gradient indexlens component which satisfies the conditions (1) and (2).

The forty-fourth embodiment has a composition illustrated in FIG. 44, oris composed of a positive lens component, a positive lens component anda positive lens component in that order from the object side. Speakingmore concretely, the forty-fourth embodiment is composed, in order fromthe object side, of a first lens unit which consists of a positivemeniscus lens component having a concave surface on the object side anda positive biconvex lens component, an aperture stop and a second lensunit which consists of a positive concavo-planar lens component: thelens component disposed on the image side being a radial type gradientindex lens component.

The forty-fourth embodiment is an example where a first lens unit iscomposed of two lens components for sharing a refractive power betweenthe two lens components, in contrast to the first lens unit used in theforty-third embodiment, thereby correcting aberrations more favorably inthe lens system as a whole.

Each of the forty-fifth through forty-ninth embodiments uses areflecting surface in a lens system and has the fourth compositionaccording to the present invention as described below:

The forty-fifth embodiment has a composition illustrated in FIG. 45, oris composed of a negative lens component, a positive lens component anda positive lens component, in that order from the object side. Speakingmore concretely, the forty-fifth embodiment is composed, in order fromthe object side, of a first lens unit which consists of a negative lenselement having a concave surface on the image side and a positive lenscomponent having a reflecting surface, an aperture stop S, and a secondlens unit consisting of a radial type gradient index lens componentwhich has two planar surfaces (the lens component having both planarsurfaces r₂ and r₈, which are shown in FIG. 45) and a positiverefractive power.

The forty-fifth embodiment is an example of a thin lens system which isthinned in the x direction, as shown in FIG. 51D by using a reflectingsurface R.

Asymmetry of a lens system as a whole is moderated by using a reflectingsurface since the lens system has more or less latitude in a totallength thereof. For favorably correcting lateral chromatic aberrationwhen at least one reflecting surface is to be used, it is desirable toconfigure a radial type gradient index lens component so as to satisfythe following condition (20):

    -0.02<1/V.sub.1 <0.01                                      (20)

If the upper limit of 0.01 of the condition (20) is exceeded, lateralchromatic aberration will be undercorrected. If the lower limit of -0.02of the conditon (20) is not satisfied, in contrast, lateral chromaticaberration will undesirably be overcorrected.

For favorably correcting lateral chromatic aberration and a Petzval'ssum when at least one reflecting surface is to be used, it is desirableto configure a radial type gradient index lens component so as tosatisfy the following condition (21):

    -0.3<N.sub.1 ×f.sup.2 <-0.03                         (21)

If the upper limit of -0.03 of the condition (21) is exceeded, lateralchromatic aberration and a Petzval's sum will undesirably beundercorrected. If the lower limit of -0.3 of the condition (21) is notsatisfied, in contrast, a Petzval's sum will undesirably beovercorrected.

Further, the forty-fifth embodiment allows its optical performance to beinfluenced little due to eccentricity and features high workabilitysince the radial type gradient index lens component has two planarsurfaces. The forty-fifth embodiment can be focused onto an objectlocated at an extremely short distance by moving the first lenscomponent toward the object side along an optical axis. The radial typegradient index lens component can be cemented to a protective glassplate C for an image pickup device. Furthermore, it is possible to omitthe protective glass plate C by using the radial type gradient indexlens component as a protective glass plate.

A transparent member which has a function of a reflecting surface and afunction of a lens can be manufactured by using a transparent materialsuch as a plastic material, a glass material, a crystal or a liquid.Though an image formed by using a reflecting surface is inverted ascompared with an image formed without using a reflecting surface, theinverted image can be electrically corrected. An inverted imageelectrically recorded in a memory can be corrected by electricallyreading it out from an opposite side.

The forty-sixth embodiment has a composition shown in FIG. 46, or iscomposed of a negative lens component, a positive lens component and apositive lens component in that order from the object side. Speakingmore concretely, the forty- sixth embodiment is composed, in order fromthe object side, of a first lens unit which consists of a negative lenscomponent, a reflecting surface and a positive biconvex lens component,an aperture stop, and a second lens unit which consists of a positivemeniscus lens component having a concave surface on the object side: thelens component disposed on the image side being a radial type gradientindex lens component. Distortion, in particular, is corrected favorablyby configuring the image side surface of the first lens component as anaspherical surface.

The forty-sixth embodiment is an example of a lens system which isthinned as shown in FIGS. 51A, 51B, 51C and 51D by using a reflectingsurface.

For obtaining favorable imaging performance when at least one reflectingsurface is to be used, it is desirable to configure a radial typegradient index lens component so as to satisfy the following condition(22):

    2<t.sub.G /f<7                                             (22)

When the condition (22) is satisfied, it is possible to favorablycorrect lateral chromatic aberration without lowering transmittance in aradial type gradient index lens component or aggravating flare. If thelower limit of 2 of the condition (22) is not satisfied, lateralchromatic aberration will be undercorrected. If the upper limit of 7 ofthe condition (22) is exceeded, production of flare or lowering oftransmittance will undesirably pose a problem.

The forty-seventh embodiment has a composition shown in FIG. 47, or iscomposed of a negative lens component and a positive lens component, inthat order from the object side. In more detail, the forty-seventhembodiment is composed, in order from the object side, of a first lensunit which consists of a negative biconcave lens component having areflecting surface R, an aperture stop S, a second lens unit whichconsists of a radial type gradient index lens component having twoplanar surfaces and a reflecting surface R₁. The forty-seventhembodiment is an example wherein an image pickup surface is made to beparallel with an object surface by disposing the reflecting surface R₁on the image side of the object side lens component.

When the lens system according to the present invention which comprisesat least one reflecting surface is to be used in a system requiringhigher imaging performance, it is desirable to configure a radial typegradient index lens component so as to satisfy the following condition(23):

    -0.1<N.sub.2 ×f.sup.4 <0.1                           (23)

If the lower limit of -0.1 of the condition (23) is not satisfied, comawill undesirably be undercorrected. If the upper limit of 0.1 of thecondition (23) is exceeded, in contrast, coma will undesirably beovercorrected.

The forty-eighth embodiment has a composition illustrated in FIG. 48, oris composed of a positive lens component and a positive lens component.Speaking more detailedly, the forty-eighth embodiment is composed, inorder from the object side, of an aperture stop S, a positive biconvexlens component having a reflecting surface R, and a radial type gradientindex lens component having two planar surfaces. The forty-eighthembodiment in which the aperture stop is disposed on the object side ofthe reflecting surface facilitates the manufacture of a stop mechanism.

The forty-ninth embodiment has a composition shown in FIG. 49, or iscomposed of a negative lens component, a positive lens component and apositive lens component, in that order from the object side. Speakingmore concretely, the forty-ninth embodiment is composed, in order fromthe object side, of a negative lens component having a concave surfaceon the image side, a positive lens component having a reflecting surfaceR, a stop S, and a radial type gradient index lens component having twoplanar surfaces and a positive refractive power. The forty-ninthembodiment is an example wherein distortion, in particular, is correctedfavorably by configuring the image side surface of the first lenscomponent as an aspherical surface.

In the forty-ninth embodiment, the lens system is composed of a masterlens portion M and an interchangeable lens portion A. A lens systemwhich has two focal points can easily be obtained by exchanging theinterchangeable lens portion A with a reflecting surface R as shown inFIG. 50. Further, a lens system which has a plurality of focal pointscan be obtained by preparing a plurality of interchangeable lensportions and using these lens portions in exchange. Since the masterlens portion M is usable coonly to all interchangeable lens portions, itcan be prepared at a low cost. The interchangeable portion A has arefractive power of nearly zero so that a focal light bundle is incidentonto a second lens unit.

FIGS. 52A and 52B show a means for exchanging the interchangeable lensportions. FIGS. 52A and 52B are sectional views illustrating the lenssystem according to the present invention which is used in an imagepickup apparatus such as a camera as seen from the object side. In thesedrawings, a reference numeral 11 represents a frame of the image pickupapparatus, a reference numeral 12 designates an optical axis, areference numeral 13 denotes a master lens, reference numerals 14 and 15represent interchangeable lenses A and B, and a reference numeral 16designates a mechanism for exchanging the interchangeable lenses A and Bwith each other. FIG. 52A shows a lens system which is composed of acombination of the master lens and the interchangeable lens A. FIG. 52Bshows a condition in which the exchanging mechanism is moved upward forcombining the master lens with the interchangeable lens 15. A lenssystem having two focal points can be obtained by exchanging theinterchangeable lenses as described above.

Another means can be obtained, for example, as shown in FIGS. 53A, 53Band 53C. FIG. 53A shows a lens system according to the present inventionwhich is used in an image pickup apparatus such as a camera as seen fromthe object side. In FIGS. 53A, 53B and 53C, a reference numeral 11represents a frame of the image pickup apparatus, a reference numeral 12designates an optical axis, a reference numeral 13 denotes a master lensportion, reference numerals 14, 15, 16 and 17 represent interchangeablelenses A, B, C and D, a reference numeral 18 designates a mechanism forexchanging the interchangeable lenses A, B, C and D, a reference numeral19 denotes a reflecting surface, and a reference numeral 20 represents arotating shaft of the exchanging mechanism. FIG. 53B shows a lens systemwhich is composed of a combination of the master lens and theinterchangeable lens A, whereas FIGS. 53C shows a condition in which alens system is composed of a combination of the master lens and theinterchangeable lens B by turning the exchanging mechanism 18 at anangle of 90° around the rotating shaft 20. A lens system which has fourfocal points can be obtained by exchanging the interchangeable lenses asdescribed above.

A fiftieth embodiment has a composition illustrated in FIG. 54, or is azoom lens system which is composed, in order from the object side, of afirst lens unit having a negative refractive power, a second lens unithaving a positive refractive power, a third lens unit having a negativerefractive power and a fourth lens unit having a positive refractivepower. The first lens unit and the fourth lens unit are kept stationaryduring a change of a magnification, whereas the second lens unit and thethird lens unit are movable for the change of the magnification. Thefirst lens unit is composed of a negative lens component and a positivelens component in order from the object side, and has a function to leadaxial and offaxial light bundles to the second lens unit. The secondlens unit is composed of a single positive lens component and has avari-focal function, or is moved along an optical axis for changing themagnification. Since the second lens unit is composed of a radial typegradient index lens component, it corrects aberrations produced in thislens unit, thereby being capable of reducing variations of aberrationsdue to the change of the magnification. The third lens unit is composedof a single negative lens component and helps to correct a deviation ofan image location caused by changing magnification. The fourth lens unitis composed of a single positive lens component and leads the lightbundles from the third lens unit to an image surface. Two plane parallelplates disposed subsequently to the fourth lens unit represent filtersfor cutting off components having specific wavelengths. These filtersare, for example, a low pass filter and an infrared cut filter. A planeparallel plate disposed on the image side represents a cover glass platefor an image pickup device. An aperture stop is disposed on the objectside of the radial type gradient index lens component and is movedtogether with the radial type gradient index lens component for changingthe magnification.

In FIG. 54, the reference symbol W represents a wide position, thereference symbol S designates an intermediate focal length and thereference symbol T denotes a tele position.

It is desirable for a zoom lens system that aberrations are favorablycorrected in each of the lens units for reducing variations ofaberrations caused by changing a magnification. It is difficult tocorrect aberrations in particular in a lens unit which has a vari-focalfunction and tends to have a refractive power stronger than that of anyone of the other lens units. By using a radial type gradient index lenscomponent which is capable of favorably correcting chromatic aberrationand the other aberrations in a lens unit having a vari-focal function,it is possible to reduce variations of aberrations to be caused bychanging a magnification. Further, the cost of polishing is reduced byconfiguring the radial type gradient index lens component so as to havetwo planar surfaces. Needless to say, aberrations can be corrected morefavorably by configuring the radial type gradient index lens componentso as to satisfy at least one of the conditions (1), (2), (3), (4), (5),(6), (7), (8), (9), (10), (11), (18), (19), (20), (21), (22) and (23).

Though the fiftieth e ent is composed of lens components in a number assmall as five, aberrations are corrected favorably and variations ofaberrations caused by changing a magnification are small in this lenssystem.

The lens system according to the present invention is composed of lenscomponents in an extremely small number of the order of two to five, andnevertheless has a compact size and favorably corrected aberrations.

What is claimed is:
 1. A lens system consisting of only four lens units,said lens system comprising, in order from an object side:a first lensunit; a second lens unit having a positive refractive power; a thirdlens unit having a negative refractive power; and a fourth lens unithaving a positive refractive power, wherein only a space between saidfirst lens unit and said second lens unit, a space between said secondlens unit and said third lens unit, and a space between said third lensunit and said fourth lens unit are changed when a magnification changeis performed from a wide angle position to a telephoto position, whereinsaid second lens unit comprises at least one radial type gradedrefractive distribution lens, and wherein said first lens unit is fixedon an optical axis from said wide angle position to said telephotoposition.
 2. A lens system according to claim 1 wherein said first lensunit has a negative refractive power.
 3. A lens system according toclaim 1 wherein said radial type gradient index lens component has twoplanar surfaces.
 4. A lens system according to claim 1 wherein saidsecond lens unit and said third lens unit are moved along an opticalaxis for changing a magnification.
 5. A lens system according to claim1, further comprising a filter disposed between said fourth lens unitand an image surface.
 6. A lens system according to claim 1, furthercomprising an aperture disposed between said first lens unit and saidsecond lens unit.
 7. A lens system according to claim 1, wherein alllenses belonging to said first lens unit are homogeneous lenses.
 8. Alens system according to claim 1, wherein all lenses belonging to saidthird lens unit are homogeneous lenses.
 9. A lens system according toclaim 1, wherein said first, third and fourth lens units consist ofhomogeneous lenses.
 10. A lens system consisting of only four lensunits, said lens system comprising, in order from an object side:a firstlens unit; a second lens unit having a positive refractive power; athird lens unit having a negative refractive power; and a fourth lensunit having a positive refractive power, wherein only a space betweensaid first lens unit and said second lens unit, and a space between saidsecond lens unit and said third lens unit, and a space between saidthird lens unit and said fourth lens unit are changed when amagnification change is performed from a wide angle position to atelephoto position, wherein said second lens unit comprises at least oneradial type graded refractive distribution lens, and wherein said firstlens unit is composed of a plurality of lenses with an air spacereserved therebetween.
 11. A lens system according to claim 10, whereinsaid first lens unit has a negative refractive power.
 12. A lens systemaccording to claim 10, wherein said radial type gradient index lenscomponent has two planar surfaces.
 13. A lens system according to claim10, wherein said second lens unit and said third lens unit are movedalong an optical axis for changing a magnification.
 14. A lens systemaccording to claim 10, further comprising a filter disposed between saidfourth lens unit and an image surface.
 15. A lens system according toclaim 10, further comprising an aperture disposed between said firstlens unit and said second lens unit.
 16. A lens system according toclaim 10, wherein all lenses belonging to said first lens unit arehomogeneous lenses.
 17. A lens system according to claim 10, wherein alllenses belonging to said third lens unit are homogeneous lenses.
 18. Alens system according to claim 10, wherein said first, third and fourthlens units consist of homogeneous lenses.
 19. An image pickup systemhaving a lens system consisting of only four lens units, said lenssystem comprising in order from an object side:a first lens unit; asecond lens unit having a positive refractive power; a third lens unithaving a negative refractive power; and a fourth lens unit having apositive refractive power, wherein only a space between said first lensunit and said second lens unit, a space between said second lens unitand said third lens unit, and a space between said third lens unit andsaid fourth lens unit are changed when a magnification change isperformed from a wide angle position to a telephoto position, whereinsaid second lens unit comprises at least one radial type gradedrefractive distribution lens, and wherein said first lens unit is fixedon an optical axis from said wide angle position to said telephotoposition.
 20. An image pickup system according to claim 19, wherein saidfirst lens unit has a negative refractive power.
 21. An image pickupsystem according to claim 19, wherein said radial type gradient indexlens component has two planar surfaces.
 22. An image pickup systemaccording to claim 19, wherein said second lens unit and said third lensunit are moved along an optical axis for changing a magnification. 23.An image pickup system according to claim 19, further comprising afilter disposed between said fourth lens unit and an image surface. 24.An image pickup system according to claim 19, further comprising anaperture disposed between said first lens unit and said second lensunit.
 25. An image pickup system according to claim 19, wherein alllenses belonging to said first lens unit are homogeneous lenses.
 26. Animage pickup system according to claim 19, wherein all lenses belongingto said third lens unit are homogeneous lenses.
 27. An image pickupsystem according to claim 19, wherein said first, third and fourth lensunits consist of homogeneous lenses.
 28. An image pickup system having alens system consisting of only four lens units, said lens systemcomprising in order from an object side:a first lens unit; a second lensunit having a positive refractive power; a third lens unit having anegative refractive power; and a fourth lens unit having a positiverefractive power, wherein only a space between said first lens unit andsaid second lens unit, and a space between said second lens unit andsaid third lens unit, and a space between said third lens unit and saidfourth lens unit are changed when a magnification change is performedfrom a wide angle position to a telephoto position, wherein said secondlens unit comprises at least one radial type graded refractivedistribution lens, and wherein said first lens unit is composed of aplurality of lenses with an air space reserved therebetween.
 29. Animage pickup system according to claim 28, wherein said first lens unithas a negative refractive power.
 30. A image pickup system according toclaim 28, wherein said radial type gradient index lens component has twoplanar surfaces.
 31. A image pickup system according to claim 28,wherein said second lens unit and said third lens unit are moved alongan optical axis for changing a magnification.
 32. An image pickup systemaccording to claim 28, further comprising a filter disposed between saidfourth lens unit and an image surface.
 33. An image pickup systemaccording to claim 28, further comprising an aperture disposed betweensaid first lens unit and said second lens unit.
 34. An image pickupsystem according to claim 28, wherein all lenses belonging to said firstlens unit are homogeneous lenses.
 35. An image pickup system accordingto claim 28, wherein all lenses belonging to said third lens unit arehomogeneous lenses.
 36. An image pickup system according to claim 28,wherein said first, third and fourth lens units consist of homogeneouslenses.